Department of Health Sciences, Faculty of Medicine, Lund University, Sweden, 2009

PREGNANCY AND LEISURE TIME PHYSICAL ACTIVITY

Hanne Kristine Hegaard

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Copyright © by Hanne Kristine Hegaard ISSN 1652-8220 

ISBN 978-91-86253-82-0 Lund University, Faculty of Medicine Doctoral Dissertation Series 2009:94Printed in Sweden by Media-Tryck

 

CONTENTS

PREGNANCY AND LEISURE TIME PHYSICAL ACTIVITY...........1

ABSTRACT.................................................................................................9

ABBREVIATIONS....................................................................................11

ORIGINAL PAPERS................................................................................13

INTRODUCTION.....................................................................................15

BACKGROUND........................................................................................17

Leisure time physical activity and nonpregnant women................................................17

Preterm delivery.................................................................................................................18Leisure time physical activity and preterm delivery.......................................................18

Birth weight........................................................................................................................21Leisure time physical activity and birth weight...............................................................21

Pre-eclampsia.....................................................................................................................23Leisure time physical activity during pregnancy and pre-eclampsia..............................23Leisure time physical activity prior to pregnancy and pre-eclampsia.............................24

Leisure time physical activity before and during pregnancy.........................................26Pre-pregnancy leisure time physical activity...................................................................26Patterns of leisure time physical activity during pregnancy............................................27Barriers or stimulators for leisure time physical activity during pregnancy...................28

AIMS...........................................................................................................30

STUDY POPULATION AND METHODS.............................................32

Design..................................................................................................................................32

Study population................................................................................................................33Aarhus Birth Cohort - Papers I and II..............................................................................34

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Smoke-free Newborn study -Paper III.............................................................................34Danish Dystocia study -Paper IV....................................................................................35

Methods...............................................................................................................................35

STATISTICAL ANALYSIS.....................................................................40

Content Analysis................................................................................................................41

ETHICAL CONSIDERATIONS.............................................................42

RESULTS...................................................................................................44

Leisure time physical activity and preterm delivery......................................................44

Sports and leisure time physical activity during pregnancy and mean birth weight.. 45

Sports and leisure time physical activity and delivery of infants with low birth weight or high birth weight, respectively.....................................................................................46

Leisure time physical activity before pregnancy and pre-eclampsia............................47

Patterns of leisure time physical activity before and during pregnancy......................47Level of leisure time physical activity during pregnancy related to pre-pregnancy leisure time physical activity.......................................................................................................47

Experiences and views of leisure time physical activity during pregnancy among nulliparous women.............................................................................................................49

Physical activity as a lifestyle..........................................................................................49Body awareness...............................................................................................................49Carefulness......................................................................................................................50Sense of benefit...............................................................................................................51

DISCUSSION.............................................................................................52

Methodological issues........................................................................................................52Quantitative studies.........................................................................................................52Qualitative study..............................................................................................................57

Discussion of findings........................................................................................................58

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CONCLUSIONS........................................................................................62

FUTURE PERSPECTIVES......................................................................64

SUMMARY IN DANISH..........................................................................66

ACKNOWLEDGEMENTS......................................................................70

REFERENCES............................................................................................72

APPENDIX

PAPER I - V

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ABSTRACTAims: To examine associations between leisure time physical activity before and during pregnancy and preterm delivery, birth weight, and pre-eclampsia and to describe levels of physical activity prior to and during pregnancy. Furthermore to elucidate women’s experiences and views of leisure time physical activity during pregnancy in a qualitative study. Methods: Data sources were: the Aarhus Birth Cohort (1989-1991), the Smoke-free Newborn Study (1996-1998), and the Danish Dystocia Study (2004-2005) including from 2750 to 5750 pregnant women in each study population. Questionnaires provided information about leisure time physical activity (sedentary, light, moderate-to-heavy and competitive sports) and sports. Findings: Pregnant women engaged in moderate-to-heavy leisure time physical activity during pregnancy had a 66% significantly reduced risk of preterm delivery (adjusted OR=0.34, 95% CI:0.14-0.85), while women with light leisure time physical activity had an unchanged risk compared to sedentary women. Practicing more than 1 type of sports was associated with a 91% significantly reduced risk of preterm delivery. Women performing sports 1-2 hours, or 3 hours or more weekly during pregnancy delivered infants with similar birth weight, and had similar risk of infants with low (<2500g) or high birth weight (>4500g) as sedentary women. Similar findings were found when leisure time physical activity was studied. Women engaged in leisure time physical activities prior to pregnancy had the same risk of pre-eclampsia as sedentary women. From prior to pregnancy to the third trimester women’s level of leisure time physical activity decreased. Women with the highest level of leisure time physical activity before pregnancy remained the most physically active in leisure time during pregnancy. The qualitative study indicated that pregnant women desired to continue to be physically active in leisure time. Pregnancy-related discomforts/complications, a growing body and senses of insecurity with physically activity during pregnancy were barriers to maintain previous levels of physical activity. The women experienced that it was most often possible to overcome these barriers and continue to be active, and felt enjoyment and physical well-being from doing this.Conclusions: This thesis contributes new knowledge on the subject by demonstrating that pregnant women performing leisure time physical activity with the highest intensity had the lowest risk of preterm delivery. No associations between leisure time physical activity and offspring birth weight or risk of pre-eclampsia were found. It is a new dimension that pregnant women described their experiences in overcoming barriers in order to continue to be psychically active.The overall conclusion is that, that performing sports or higher level of leisure time physical activity prior to pregnancy or during pregnancy is not associated with disadvantages; rather there are some psychological and physical benefits for the mother and her offspring.

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ABBREVIATIONS

ACOG American College of Obstetricians and GynecologistsANOVA The analysis of varianceBMI Body mass indexCI Confidence intervale.g. Exempli gratiaLTPA Leisure time physical activityN/n NumberNo. NumberNS Not significantOR Odds ratioSD Standard deviationSPSS Statistical Package for the Social SciencesWHO World Health Organization

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ORIGINAL PAPERS

This thesis for the degree of Doctoral is based on the following five papers referred to in the text by their Roman numerals:

I HK Hegaard, M Hedegaard, P Damm, B Ottesen, K Petersson, TB Henriksen. Leisure time physical activity is associated with reduced risk of preterm delivery. Am J Obstet Gynecol. 2008 Feb;198(2):180.e1-5.

II HK Hegaard, K Petersson, M Hedegaard, B Ottesen, AK Dykes, TB Henriksen, P Damm. Sports and leisure time physical activity in pregnancy and birth weight. A population-based study. Scand J Med Sci Sports 2009 Apr 14. Epub ahead of print.

III HK Hegaard B Ottesen, TB Henriksen, K Petersson, P Damm, AK Dykes.The association between leisure time physical activity in the year before pregnancy and pre-eclampsia. (Accepted Journal of Obstetrics and Gynaecology).

IV HK Hegaard, P Damm, M Hedegaard, TB Henriksen, B Ottesen, AK Dykes, H Kjaergaard.Sports and leisure time physical activity during pregnancy in nulliparous women. (Submitted).

V HK Hegaard, H Kjaergaard, K Petersson, P Damm, AK Dykes.Experiences of physical activity during pregnancy in Danish nulliparous women with a physically active life before pregnancy. Interviews and content analysis. (Submitted).

All papers have been reprinted with the kindly permission of the publishers.

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INTRODUCTIONThe impact of physical activity on public health in general has attracted much attention in recent years. Regular physical activity in nonpregnant individuals reduces the risk of many different diseases (1-4). Given the important health benefits of leisure time physical activity for nonpregnant women, it would seem reasonable to assume that recreational physical activity also positively affects pregnancy outcome, but it has been questioned whether this is safe (5).

The American guidelines on exercise during pregnancy from 1994 (6) recommended caution but in 2002 the American College of Obstetricians and Gynecologists (ACOG) issued new guidelines on exercise during pregnancy as well as the postpartum period (7), stating on page 79:

“The current Centers for Disease Control and Prevention and American College of Sports Medicine recommendation for exercise, aimed at improving the health and well-being of nonpregnant individuals, suggests that an accumulation of 30 minutes or more of moderate exercise a day should occur on most, if not all, days of the week. (8) In the absence of either medical or obstetric complications, pregnant women also can adopt this recommendation.”

ACOG has thus decided to recommend that healthy women with normal pregnancies follow the American recommendations made in regard to all nonpregnant adults in 1995 (9) although pregnant women were cautioned against some types of sports. Walking briskly was described as moderate-intensity physical activity. In 2004 the National Board of Health of Denmark likewise recommended 30 minutes a day (9) of moderate physical activity (Borg scale 12-13) for all healthy pregnant women with an anticipated normal birth. Women at increased risk of pre-eclampsia or gestational diabetes should be even more active. The Borg scale 12-13 corresponds to experiencing physical activity as fairly light to somewhat hard (10). Although American and Danish recommendations on physical activity during pregnancy were published, literature on this subject was sparse at that time. A meta-analysis called for more studies regarding birth weight and preterm delivery,(11) and a Cochrane meta-analysis (11;12), “Aerobic exercise for women during pregnancy”, concluded that the available data were inadequate as evidence of important risks or benefits for mothers and infants. The association between leisure time physical activity and outcomes such as preterm delivery, birth weight and pre-eclampsia need more attention. Likewise we must acquire more understanding of women’s own experiences and views of physical activity during pregnancy. This has been explored in very few studies (13-15).

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BACKGROUNDThis thesis is about leisure time physical activity. Caspersen wrote in 1985, page 26, (16) “Physical activity is defined as any bodily movement produced by skeletal muscles that results in energy expenditure. The energy expenditure can be measured in kilocalories. Physical activity in daily life can be categorized into occupational, sports, conditioning, household, or other activities.”

Based upon the above, leisure time physical activity in this thesis is defined as: any bodily movement during leisure time produced by skeletal muscles that results in energy expenditure. Leisure time physical activity in daily life can be categorized as sports, conditioning, or other activities. Housework is not included in leisure time physical activity in this thesis.

Sports is a part of leisure time physical activity and is similar to exercise which Caspersen (16), page 26 defined as “… a subset of physical activity that is planned, structured, and repetitive and has as a final or an intermediate objective the improvement or maintenance of physical fitness. Physical fitness is a set of attributes that are either health- or skill-related.”

Leisure time physical activity and nonpregnant womenObservational prospective studies show that mortality and disease outcomes such as cardiovascular outcomes, type 2 diabetes, some cancers and mortality from any cause have been significantly reduced in women who perform regular physical activity compared to inactive women (1;2;4;4;17;18). Changes in women’s physical activity over time influence the risk of diseases. An increase in total physical activity score (both vigorous activity and walking) over a period has shown a reduction in the risk of cardiovascular diseases and events (19). A study found that women who increased their activities from light intensity four hours weekly to more intensive activities four hours weekly over a period of time reduced their risk of breast cancer (20), a result that was not confirmed in a study population of primarily premenopausal women (21). Women performing moderate-to-high leisure time physical activity at baseline and at 17 years follow-up had a decreased risk of mortality compared to women with low activity all the time (4). In 2007 the American recommendations on exercise for adults were updated, advising more vigorous activities (2).

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Preterm deliveryPreterm delivery is defined as birth before 37 weeks of gestation or less than 259 completed days (22). Preterm birth is a major determinant of neonatal mortality and morbidity (23-25) and also long-term sequelae (26-30). It is therefore worrying that the proportion of preterm deliveries has increased in recent years in different countries (31;32). In Denmark spontaneous preterm deliveries in primiparous women increased from 3.8% in 1995 to 5.7% in 2004. Low risk women were defined as white European primiparous females aged 20-40 with a singleton spontaneous pregnancy (31). Although the etiology for preterm births is partially known, (28), many risk factors are known. It is well known that factors such as low and high maternal age, primiparity, smoking, high alcohol intake, low socioeconomic status, psychological stress, and infection are associated with preterm delivery (23;33-40), whereas the association between occupational work and preterm delivery is more inconsistent (41-44). Some risk factors such as age, pre-pregnancy weight, or socioeconomic status cannot change during pregnancy. It is thus important to focus on potentially modifiable factors during pregnancy; leisure time physical activity is a factor, which may be modified. Earlier reviews underlined the need for more extensive studies on the association between leisure time physical activity and preterm delivery (5;11).

Leisure time physical activity and preterm delivery

As noradrenalin stimulates uterine activity (45) and the maternal plasma concentrations of noradrenalin and adrenalin are increased after a short period of exercise (46) there has been concern that exercise could increase the frequency of uterine concentrations and cause preterm delivery. However, a study of 81 low risk women found an increase in uterine activity only after walking and climbing stairs in late pregnancy and not in women performing organized physical exercise (47) and no increase in uterine activity was seen after walking or bicycling in a study of 17 women in the last 8 weeks of pregnancy (48).

In a case-control study with 175 cases and 313 controls, Berkowitz et al. (49) found that pregnant women participating in leisure time physical activity halved their risk of delivering preterm (OR=0.53, 95% CI:0.36-0.78) compared to less active women.

In a follow-up study of 557 pregnant women, the association between weekly average energy expenditure during leisure time physical activity and risk of preterm delivery was examined (50). Physical activity level was categorized as any physical activity, low-moderate (defined as <1000 kcal of energy expenditure weekly) and heavy leisure time (defined as >1000 kcal of energy expenditure

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weekly). Low-moderate leisure time physical activity was not associated with preterm delivery, while women performing heavy leisure time physical activity had a reduced risk of preterm delivery (OR=0.11, 95% CI:0.02-0.81).

In a population-based study from Brazil of 4174 women the association between any leisure time physical activity and preterm delivery was examined (51). Women engaged in any leisure time physical activity during pregnancy experienced a significantly reduced risk of preterm delivery (RR=0.55, 95% CI:0.32-0.96) compared to women not engaged in leisure time physical activity. In this study leisure time physical activity was defined as more than 90 minutes of activity per week whether at work, in the home or at school.

Evenson et al. examined the association between vigorous leisure time physical activity during first and second trimester and the risk of preterm delivery in a prospective study of 1699 women (52). Vigorous leisure time physical activity was defined as e.g. aerobic exercise, jogging, lap swimming, and moderate to fast bicycling two times weekly. Women performing vigorous leisure time physical activity during the first or second trimester did not show an increased risk of preterm delivery (OR=0.80, 95% CI:0.48-1.35) and (OR=0.52, 95% CI:0.24-1.11).

In a study from the Danish National Birth Cohort with 87232 singleton pregnancies (53) a reduced risk of preterm delivery was found in women performing some kind of exercise during pregnancy compared to sedentary women (hazard ratio=0.83; 95% CI:0.76-0.88). However, no dose-response was found. Exercise was defined in the study as brisk walking and different types of sports.

The previously mentioned Cochrane review of aerobic exercise and preterm delivery concluded that there was some evidence that aerobic exercise did not affect the length of pregnancy (12).

Earlier epidemiological studies have focused on vigorous sports training several times a week during pregnancy and this was associated with a decreased (49-51;53;54) or unchanged risk of preterm delivery (52;55;56).

A review of studies on leisure time physical activity and preterm delivery shows that pregnant women who are physically active during pregnancy have either an unchanged or reduced risk of preterm delivery. This seems to apply regardless of whether the results are from prospective or case-control studies, or from what country the study population derives. Table 1 gives an overview of the selected studies about leisure time physical activity and preterm delivery.

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Table 1.Overview of selected studies about leisure time physical activity and preterm deliveryAuthor Design

NMeasure of physical activity Adjusted risk,

95% CI Evenson, 2002 (52)

Prospective studyN=1699

Vigorous activity 1.trimester* < 1 time weekly > 2 times weeklyVigorous activity 2.trimester < 1 time weekly > 2 times weekly

1.00 ref0.80 (0.48-1.35)

1.00 ref0.52 (0.24-1.11)

Hatch, 1998 (50)

Cohort study N=557

Exercise during pregnancy Non exercisers Low-moderate exercise Heavy exercise

1.00 ref 1.11 (0.88-1.39)0.11 (0.02-0.81)

Juhl,2007 (53)

Cohort studyN=87232

Hours exercise per week 0>0-< 1>1-< 2>2-< 3>3-< 5>5

1.00 ref0.82 (0.72-0.87)0.86 (0.72-0.92)0.89 (0.76-1.05)0.89 (0.75-1.06)0.81 (0.64-1.04)

Misra,1998 (54)

Cohort study Exercise (times in first and second trimester)< 60> 60

1.00 ref0.55 (0.26-1.14)

Domingues, 2008 (51)

Cohort studyN=4147Retrospective collected data.

LTPA > 90/min 1.trimesterNoYesLTPA > 90/min in 2. trimesterNoYesLTPA > 90/min in 3. trimesterNoYes

1.00 ref0.92 (0.67-1.26)

1.00 ref 0.89 (0.61-1.29)

1.00 ref0.58 (0.34-0.98)

Berkowitz,1983 (49)

Case-controlCases 175Controls 313

LTPA during pregnancyNoYes

1.00 ref0.53 (0.36-0.78)

*Vigorous activity is e.g aerobics or jogging. ** LTPA: Leisure time physical activity.

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Birth weightBirth weight is an important predictor of survival and health in the perinatal period and later in life. Infants with a low birth weight (<2500g) have an increased risk of perinatal morbidity and mortality (57;58) and infants with a high birth weight (defined as >4000g) are more likely to have brachial palsy, meconium aspiration, shoulder dystocia, clavicular fracture and low Apgar score (59-62). Later in life low infant birth weight is associated with educational disadvantage (26), and an increased risk in adulthood of hypertension as well as type 2 diabetes (63). High birth weight has been associated with increased risk of diseases in adulthood e.g. type 2 diabetes, and, in women, breast cancer (64;65). Maternal height, pre-pregnancy BMI, weight gain, education, parity, neonatal sex, gestational age, maternal disorders, maternal smoking and alcohol intake are some of the factors that influence birth weight (66-69). Previous literature reviews called for more extensive studies of the link between leisure time physical activity and birth weight (5;11).

Leisure time physical activity and birth weight

For many years it has been questioned whether maternal leisure time physical activity during pregnancy affects infant birth weight. As the redistribution of blood flow to the skin and working muscles during exercise might reduce uterine blood flow (70) fetal growth could theoretically be compromised. The reduction in blood flow and thereby in oxygen and nutrient delivery during exercise is compensated by an increase in the volume of maternal plasma, an increase in intervillous space blood volume and in the size of placenta and in maternal cardiac output in women who exercise (71). This relationship does not apply to all forms of exercise nor exercise at different times during the pregnancy (71). One study indicates that regular leisure time physical activity modifies placental development primarily in early and mid-pregnancy (72).

Hence, many studies have focused on the potential adverse effects on birth weight of maternal leisure time physical activity during pregnancy (73-90). Studies found that women practicing leisure time physical activity during pregnancy gave birth to infants with either a higher birth weight (80;89;91), similar birth weight (56;73;76;79;81;84;92), or reduced birth weight (74;75;77;78;82) compared to infants born of more inactive pregnant women. It is difficult to compare the results of these studies, as the study populations varied from 50 to over 20,000 women and methods of analysis, including confounders, are very different. Furthermore, the women’s pre-pregnancy activity levels varied, as did intensity, frequency and duration of physical activity during pregnancy, and this can affect infant birth

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weight (9). Finally, the numbers of weeks during which women exercised during pregnancy and at what points during pregnancy they exercise also varied (92). The association between leisure time physical activity and mean birth weight are be reported by means of three different meta-analyses. The first meta-analysis was based on 18 studies (93) and sought to examine whether it was safe for the mother and child that the mother exercised during pregnancy. The study concluded that healthy pregnant women who followed an exercise program whereby they trained 3 times a week for 45 minutes maintaining a pulse below 144 beats per minute did not deliver children with the same birth weight as children born of pregnant women who were less active.

A Cochrane meta-analysis of aerobic exercise for women during pregnancy based on 11 randomised studies involving 472 women concluded that the findings as to the effect of exercise on mean birth weight were inconsistent (10).

A third and more extensive meta-analysis based on 30 studies both observational and randomized studies evaluated the effect of exercise on birth weight (9). The women’s pre-pregnancy activity levels varied, as did intensity, frequency and duration of physical activity during pregnancy, and it was evaluated if this affected infant birth weight (9). The overall conclusion was that exercise during pregnancy does not have a noticeable effect on the mean birth weight except when women continue vigorous endurance exercise 3 times per week or more during the third trimester. Thus, continuing to practice vigorous endurance exercise during the third trimester was associated with infants born with a lower mean birth weight.

Few large studies have examined the association between maternal leisure time physical activity and offspring birth weight <2500g (low birth weight) or >4000g (high birth weight). In a study of 23,091 women no differences were seen in the proportion of infants born with a birth weight <2500g in women practicing moderate or vigorous levels of physical activity compared to women practicing a light level of physical activity; levels of physical activity were not clearly defined (74). An unchanged risk of delivering an infant born with a birth weight <2500g was found in women exercising during pregnancy compared to women not exercising during pregnancy (56). In a study of 9089 women it was found that women not practicing regular physical activity before and during pregnancy had a significantly higher risk of delivering an infant with a very low birth weight (<1500g) and an unchanged risk of delivering an infant with low birth weight (<2500g) than women who practiced regular physical activity before and during pregnancy (55). Thus, studies on leisure time physical activity and birth weight showed that in the majority of studies women who engaged in leisure time physically active in their leisure time gave birth to children with the same birth weight as others. Some

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studies described a slightly higher birth weight in children born of women who were physically active in their leisure time in relation to those who were inactive. Finally, some studies showed that women who continued hard exercise or vigorous physical activity in the third trimester gave birth to children with lower birth weights than those who were engaged in lighter exercise or were sedentary. However, the latter studies were largely based on studies of small populations of selected groups.

Pre-eclampsiaPre-eclampsia is one of the leading causes of maternal and perinatal mortality and morbidity worldwide and complicates from 2-7% of pregnancies in nulliparous women and 1.7% in multiparous women (94;94-96). For the fetus there is a risk of poor intrauterine growth and preterm delivery (94;95) and in mothers there is among other things the risk of placental abruption, eclampsia, coagulation disorders, acute renal or liver failure and cerebral catastrophy. Women with pre-eclampsia during pregnancy have an increased risk of cardio-vascular disease later in life (97). Well known risk factors for pre-eclampsia are e.g. nulliparity, multiple pregnancy, high pre-pregnancy BMI, chronic diseases such as diabetes, after donor insemination, and change of partner (98-102). Furthermore, a family history of type 2 diabetes and/or hypertension, hyperinsulinemia and relative insulin resistance is also a risk factor for pre-eclampsia (103-105). Smoking before pregnancy reduces the risk of pre-eclampsia (106). Little is known about whether the risk of pre-eclampsia can be modified by lifestyle changes.

Leisure time physical activity during pregnancy and pre-eclampsia

Pre-eclampsia is characterized by factors as different as endothelial dysfunction, insulin resistance, coagulation activation, and increased systemic inflammatory response, all typical findings in nonpregnant subjects with essential hypertension and type 2 diabetes (107). It is well documented that physical activity has a beneficial effect on all these factors (3) and it is therefore appropriate to study the association between physical activity and development of pre-eclampsia. As overweight women have the highest risk of pre-eclampsia (102) and are characterized by insulin resistance and increased systemic inflammatory response (98) it is furthermore relevant to examine if leisure time activity can afford any particular protection in this group.

A Cochrane review has investigated whether exercise and physical activity prevent pre-eclampsia. The review was based on two small trials of good quality. The

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authors found that there was insufficient evidence to draw reliable conclusions (95).

The association between leisure time physical activity during pregnancy and pre-eclampsia were studied in two case-control studies (108;109). One study (109) found that women practicing any recreational physical activity during pregnancy had a 35 percent reduced risk of pre-eclampsia (OR=0.65, 95% CI:0.43-0.99) compared with inactive women (109). The risk of pre-eclampsia decreased with increasing intensity and amount of energy spent on physical activity. Marcoux et al. found very similar results in an older case-control study (108;109). In a nested case-control study an unchanged risk of pre-eclampsia was seen in women practicing regular physical activity compared to others (99). In two prospective studies (110;111) from Norway and Denmark respectively, both with more than 50,000 participants, the associations between leisure time physical activity during pregnancy and pre-eclampsia were investigated. The Norwegian study (110) found a significantly reduced adjusted risk of pre-eclampsia in women exercising 25 times per month (OR=0.79, 95% CI:0.65-0.96) compared to inactive women. The Danish study (111) found an unchanged risk of pre-eclampsia in exercising pregnant women compared to inactive women. Furthermore women exercising more than 270 minutes per week or more than 420 minutes per week had an increased adjusted risk of severe pre-eclampsia (OR=1.65, 95% CI:1.11-2.43), (OR=1.78, 95% CI:1.07-2.95), respectively. Finally a smaller prospective study found an unchanged risk of pre-eclampsia in women engaged in any recreational activity during pregnancy compared to inactive women (112).

Leisure time physical activity prior to pregnancy and pre-eclampsia

Few studies have examined the association between pre-pregnancy leisure time physical activity and pre-eclampsia. A case-control study showed that, in the association between leisure time physical activity in the year before pregnancy and pre-eclampsia, only women engaged in vigorous recreational physical activity before pregnancy had a lower risk of developing pre-eclampsia (109). These findings were supported by a second study examining the predictive value of the perceived exertion during physical activity and indicating that the risk reduction. (113) was more pronounced in pregnant women with a BMI above 25kg/m2 (113). Two other studies on this subject showed that those engaged in leisure time physical activity before pregnancy had the same risk of developing pre-eclampsia as women who were inactive prior to pregnancy (99;112). Table 2 gives an overview of studies about leisure time physical activity prior to pregnancy and pre-eclampsia.

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Table 2, Overview of selected studies of leisure time physical activity prior to pregnancy and pre-eclampsia.Author Design,

numberMeasure of physical activity Adjusted risk 95% CI

Sorensen, 2003 (109)

Case-control study201 Cases383 Controls

Any recreational physical activity No YesHours spent performing sports, hours weekly 0 0.1-2.2 2.3-4.3 4.4-7.3 > 7.3Maximum intensity of physical activity* Not active Light/moderate Vigorous

1.00**ref0.67 (0.42-1.08)

1.00 ref0.71 (0.40-1.25)0.66 (0.36-1.22)0.56 (0.30-1.05)0.64 (0.35-1.16)

1.00 ref0.95 (0.57-1.58)0.40 (0.23-0.69)

Saftlas, 2004 (99)

Case-controlstudy 44 Cases2422 Controls

Sports or exercise at least once per week before and during pregnancy Before no/during no Before yes/during no Before no/during yes Before yes/during yes

1.00 ref1.12 (0.48-2.61)0.56 (0.12-2.56)0.71 (0.32-1.56)

Rudra, 2005 (113)

Case-control study24 Cases470 Controls

Sub study women > 25kg/m2

Perceived exertion during recreational physical activity None to weak Moderate Strenuous Very strenuous to maximal Perceived exertion during recreational physical activity, women > 25kg/m2

None to weak Moderate Strenuous Very strenuous to maximal

1.00 ref0.54 (0.26-1-12)0.33 (0.17-0.64)0.22 (0.11-0.44)

1.00 ref0.28 (0.08-0.95)0.21 (0.07-0.69)0.12 (0.04-0.49)

Rudra, 2007(112)

Cohort study(N=2241women)

Recreational physical activity, hours weekly 0 0.01 - 2.49 2.50 - 7.00

1.000.53 (0.26-1.07)0.50 (0.27-1.03)

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> 7.00 0.63 (0.31-1.26)

Leisure time physical activity before and during pregnancyNational guidelines recommend that pregnant women should be physically active during pregnancy (7;10). To promote activity among pregnant women it is relevant to have an overview of women’s levels of leisure time physical activity before and during pregnancy.

Pre-pregnancy leisure time physical activity

It is very difficult to compare levels of pre-pregnancy leisure time physical activity, as there is considerable difference in the ways in which leisure time physical activity/exercise prior to pregnancy is defined. In some studies pre-pregnancy leisure time physical activity was defined as “any” or “no” (114;115). On the basis of this description, 14.8% were physically active in their leisure time in a study from Brazil (114) and 61% in a study from the United States. In a study from Norway defining regular exercise as vigorous recreational physical activity at least 20 minutes once a week, 50% stated they were regular exercisers (116). In a more recent study based on the Norwegian Mother and Child Cohort Study, regular exercisers were defined as those women who exercised >3 times a week (117). Regular exercise defined in this way meant that 46.4% of the women stated that they were regular exercisers prior to pregnancy (117). In an American study from 1995 (118), exercise were defined as: “exercise at least three times a week before you found out you were pregnant”, which resulted in a frequency of exercisers of 55%. Other studies defined leisure time physical activity as: “structured exercise/recreational activities” (119), “often (every day), sometimes, rarely or never” (15), “walking for exercise, light to moderate and vigorous physical activity” (120), “specific activities, monthly frequency and duration of sessions” (121). Finally, the degree of physical activity was set on the basis of a point score established through responses to several questions on physical activity (122).

Higher household incomes, higher level of education, higher age, non-smoking , normal body mass index and no previous childbirth were some of the factors characterizing the women who were defined as exercisers or physically active in their leisure time prior to pregnancy (15;119;120;123). Hakstad found in a Norwegian study that having a child had no relevance for whether or not women engaged in regular exercise before pregnancy (116). In Hakstad’s study, regular exercise was defined as vigorous recreational exercise at least 20 minutes once a week (116).

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Patterns of leisure time physical activity during pregnancy

The studies largely agreed that regardless of the definition of leisure time physical activity/exercise, the proportion of women who were physically active in their leisure time declined when they became pregnant (15;91;107;114;116-122;124). Table 3 gives an overview of selected studies about patterns of leisure time physical activity before and during pregnancy.

Table 3, Patterns of leisure time physical activity before and during pregnancy.Study Sample Physical activity

MeasurePatterns of leisure time physical activity before and during pregnancy.

Domingues2007,Brazil(114)

4471 pregnant

Any leisure time physical activity (not activities related to household tasks).

15% any leisure time physical activity before pregnancy.10% in first trimester,9% in second trimester,7% third trimester.

Evenson 2002US(125)

1699 pregnant

Vigorous leisure activity:regular or strenuous activity such as aerobic exercise or jogging at least twice a week.

22% vigorous activity before pregnancy.14% vigorous activity in first trimester.8% vigorous activity in second trimester.

Haakstad2007Norway(116)

467 pregnant

Regular exercise:vigorous recreational activity at least 20 min once a week.

81% regularly exercise before pregnancy70% regularly exercise in first trimester,64% regularly in second trimester,47% regularly in third trimester.Frequency, duration and intensity decrease.Bicycling,running, aerobic, and strength training decrease and swimming increase.

Hinton,2001US(15;15;116)

622 pregnant.

Exercise regularly: (often=daily, sometimes,rarely, or never.)

40% become less active .20% reported becoming more physically active during pregnancy.

Owe2008Norway(117)

34508 pregnant

Regular exercise:leisure time exercise at least three times per week

46% regularly exercise before pregnancy28% regularly exercise week 17,20% regular exercise week 30.Swimming increased during pregnancy.

Zhang1996US(118)

9953 pregnant

Exercise: exercise or play sports a least three times per week.

48% exercise before pregnancy42% exercise during pregnancy

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Expecting mothers exercised less than they did prior to pregnancy although a few studies showed that 6-10% of women continued to exercise 3 or 4 hours weekly in the third trimester (116;126).The intensity of physical activity decreased as the proportion of women who engaged in more vigorous physical activity decreased during pregnancy (15;116;117;119-121). Activities that were reduced were running, jogging, strength training, aerobics, bicycling, fitness training, ball games/netball, and other sports activities (15;116;117;119-121). The proportion of women who swim was found to increase from before and throughout pregnancy (116;117;121). Walking as a form of exercise was likewise seen to increase during pregnancy in some studies (114;119) while the proportion of pregnant walkers was described as unchanged or slightly decreasing in other studies (117;120). A few studies showed, however, that some pregnant women either initiated physical activity/exercise during pregnancy (114;118) or increased their activity.

Several studies have investigated what characterizes pregnant women who exercised in the first part of pregnancy. Several studies indicated that pre-pregnancy exercise was one of the strongest predictors of exercise during pregnancy (15;117;120;125). Likewise, there was general agreement that women with higher levels of education (114-117;125), higher income (114-116;127), no children (15;114;115;117;118), non-smokers (115) were often physically active in their leisure time during pregnancy. Women with pre-pregnancy overweight were less active than others (117;118). In some studies, women who were physically active in leisure time during pregnancy were characterized by being younger (125;127) while in another study they were older (118).

Barriers or stimulators for leisure time physical activity during pregnancy

Previous abortion, previous stillbirth, or multiple gestation have all been described as inhibiting or limiting factors for women to be physically active during pregnancy (117;118).A prospective study found that the main reasons for engaging in exercise during pregnancy were for fitness and enjoyment, whereas the main reasons for not exercising were tiredness, feeling too unwell, business and in late pregnancy feeling uncomfortable in relation to exercise (128). Tiredness as a reason for not exercising was also found in other studies (14;114).

The concern that certain types of sports may be hazardous during pregnancy can also influence exercise (13;128). Lack of time, lack of energy and concern for the baby and discomfort caused by activity have also been mentioned as barriers to exercise (14). A Norwegian study furthermore described nausea and pelvic girdle

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pain as factors associated with physical inactivity (117). Pregnancy-related discomfort such as vomiting led to the cessation of physical activity as well (120).On the other hand, professional advice on physical activity during pregnancy might give reasons to continue (129) and finally, belief in one’s ability to exercise (15) and intention to exercise (130) might be significant for exercise during pregnancy.

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AIMSThe overall aims of this thesis were to examine the associations between leisure time physical activity before and during pregnancy and preterm delivery, birth weight at term and pre-eclampsia and to describe level of physical activity prior to and during pregnancy. Furthermore to elucidate women’s experiences and views of leisure time physical activity during pregnancy.

Specific aims:

To examine the association between the times spent on sports activities/leisure time physical activity during pregnancy and preterm delivery (Paper I) and birth weight at term (Paper II).

To study the association between leisure time physical activity the year before pregnancy and pre-eclampsia (Paper III).

To describe levels of sports and leisure time physical activity before and during pregnancy in nulliparous women (Paper IV).

To elucidate experiences and views of physical activity during pregnancy in nulliparous women (Paper V).

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STUDY POPULATION AND METHODSThe nature of pregnancy is complex and involves biological, psychological and sociological issues. Thus it is relevant to use both qualitative and quantitative methods in studying the association between pregnancy and leisure time physical activity. In Papers I-IV hypothetic deductive methods in the quantitative scientific area were used, while in Paper V an inductive explorative study in the qualitative area was used.

DesignAn overview of the design, sample, data collection, exposures variables and data analyses used in the five papers is shown in Table 4.

Table 4. Design of the studies.

Paper

Design Sample Data collection Exposure variable

Data analysis

Paper I.

Population-based cohort study.

Pregnant women.(N=5749)

Self-completed questionnaire.Data from 16 weeks of gestation.

Sports (yes/no, types, and hours weekly).Leisure time physical activity.

χ2 test.Logistic regressionanalysis.

Paper II.

Population-based cohort study.

Pregnant women.(N=4558)

Self-completed questionnaires.Data from 16 and 30 weeks of gestation.

Sports (types and hours weekly)Leisure time physical activity

χ2 test.ANOVA.Linear and logistic regressionanalyses.

Paper III.

Prospective study.

Pregnant women.(N=2793)

Self-completed questionnaire.Data from 12-18 weeks of gestation.

Leisure time physical activity.

χ2 test.Logistic regressionanalysis.

Paper IV.

Multicentre cohort study.

Nulliparouswomen.(N=4718)

Self-completed questionnaire.Data from 37 weeks of gestation.

Leisure time physical activity.

χ2 test.

Paper V.

Qualitative inductive study.

Mothers.(n=19)

Semi-structured individual interviews 3-4 years post partum.

Content analysis.

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Study populationData sources for papers in this thesis derived from three studies, (Figure 1)

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Aarhus Birth Cohort(1989-1991)

Smoke-free Newborn Study (1996-1998)

Danish Dystocia Study(2004-2005)

Paper I:Leisure time physical activity is associated with reduced risk of preterm delivery.

Paper IISports and leisure time physical activity in pregnancy and birth weight. A population-based study.

Paper III:The association between leisure time physical activity in the year before pregnancy and pre-eclampsia.

Paper IV:Sports and leisure time physical activities during pregnancy in nulliparous women.

Paper V:Experiences of physical activity during pregnancy in Danish nulliparous women with a physically active life before pregnancy. Interviews and content analysis.

Figure 1. Overview of papers and data sources in the thesis.

Aarhus Birth Cohort - Papers I and II

All Danish-speaking pregnant women who attended routine antenatal care programme at the maternal unit in the Department of Obstetrics, Aarhus University Hospital, Denmark, in the period August 1989 to September 1991 were invited to take part in the “Aarhus Birth Cohort 1989-91”. This is the only maternity unit in the city, serving a geographical district comprising around 250,000 persons. In Denmark >99% of pregnant women attend the antenatal care programme. A few women with specific pregnancy complications such as diabetes were referred to the department from a larger geographical area. After exclusion of women with foetal death before 28 weeks of gestation and women with twin pregnancies, the cohort comprised 8719 women. More than 99% were Caucasian. On enrolment in the study at 16 weeks of gestation the women were asked to fill in the first and second questionnaires. In total, 6917 women returned these two questionnaires. After excluding women with diseases before pregnancy (n=308), pregnant women with no valid information about infant gestational age (n=18), and women without information on leisure time physical activities and sports (n=842), the final study population in Paper I comprised 5749 women.Women who responded to the two first questionnaires received a third questionnaire before the routine antenatal visit at 30 weeks of gestation. All three questionnaires were completed be 5830 women. Women with diseases before pregnancy (n=248), women with incomplete information on leisure time physical activities and sports (n=856), women who were delivered before 37 weeks of gestation (n=144), women with no information about child’s birth weight (n=20), and pregnant women with no valid information on gestational age (n=4) were excluded. Thus the final study population in Paper II comprised 4458 women.

Smoke-free Newborn study -Paper III

All pregnant women on their first prenatal visit to the Midwifery Center at the Department of Obstetrics and Gynecology at Hvidovre University Hospital, Denmark, from November 1996 to April 1998 were asked to participate in the Smoke-free Newborn Study. Exclusion criteria were: non-Danish speaking, age <18 years, gestational age >22 weeks, women with psychiatric diseases, and alcohol or drug abuse. Participants filled in one questionnaire after their first visit to the midwife between 12 and 18 weeks of gestation. For Paper III, the study population consisted of the 2980 women who responded to a questionnaire at 12-18 weeks of gestation. Excluded from the study were women with twins (n=47), women who left the area (n=89), women with an abortion (n=17), women who did not wish to continue their participation in the study (n=1), and one woman for another reason. In total 31 women gave incomplete information on their leisure

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time physical activity during the year prior to pregnancy. Thus, the final study population in Paper III comprised 2793 women.

Danish Dystocia study -Paper IV

Participants in the Danish Dystocia Study were recruited from four major university hospitals, three county hospitals, and two local district departments in Denmark. Enrolment into the study took place in the antenatal clinics at 33 gestational weeks in the period between May 2004 and July 2005. Inclusion criteria were: nulliparous, age >18 years, Danish-speaking, singleton pregnancy, no planned elective caesarean delivery or induction of labour. The study population for Paper IV consisted of pregnant women (n=4884) who responded and returned the questionnaire at 37 weeks of gestation. Thus the final study population consisted of 4718 pregnant women who answered all questions about physical activity in leisure time before and during pregnancy.

The study population for paper V was selected among women in the Danish Dystocia Study who answered questions about their physical activities in the year before they became pregnant. Of the women who described their activities in the year before pregnancy as “sports or heavy gardening for at least four hours a week”, which was defined as moderate-to-heavy leisure time physical activity, 20 women were selected. These women were selected in such a way as to encompass some who continued to practice moderate-to-heavy activities, some who reduced the level to light activities, and some who became sedentary. Furthermore, in order to have a wide sample of information, women of different ages, pregnancy BMI, smoking status, years at school, and health status were chosen. Twenty women were contacted by mail and subsequently by telephone, and invited to be interview about their experiences with and views of leisure time physical activity during pregnancy. Nineteen agreed to be interviewed.

MethodsPapers I and IIThe first questionnaire produced details about medical and obstetric history, as well as age, height, pre-pregnancy weight, alcohol consumption, and smoking habits. The second questionnaire produced details about socio-demographic factors, caffeine consumption, sports, and leisure time physical activity. The two questionnaires were completed by the woman at home;two reminders were mailed to non-responders. The third questionnaire, which was the same as the second questionnaire concerning questions about sports and leisure time physical activity, was also filled in at home and one reminder was posted by mail to non-responders.

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Each woman was asked whether she was presently taking part in sports (“yes”/”no”), what type of sport, and how much time per week she spent on this. She was also asked which of the following four categories most correctly described her level of leisure time activity: mostly sitting (reading, watching television, going to the cinema); light physical activity (more than 3 hours a week of light gardening, playing table tennis); sports or heavy gardening (more than 3 hours a week running, swimming, playing tennis, playing badminton); participating in competitive sports several times a week. Saltin and Grimby (131) developed the questions concerning leisure time physical activity and a few years later the questions were validated in relation to mean maximum oxygen uptake VO2max by Saltin (132). Women taking part in competitive sports were few, and the two categories with the highest level of activity were pooled. Hence, leisure time activity was categorized as sedentary, light, or moderate-to-heavy.

In Paper I, the validated questionnaire instrument General Health Questionnaires (133), also used in previous studies was applied (36;134).

Preterm delivery was defined as less than 259 completed gestational days according to WHO definition (22). Gestational age in Papers I and II was estimated using three methods. In around 82% of the women in the study population in Paper I, the gestational age was estimated by an early ultrasound measurement, in around 9.5% of the women from the first day in the last certain menstrual period (9.5%), and in around 8.5% by a less certain last menstrual period or an ultrasound measurement a little later than week 22 of gestation.

The questionnaires contained numerous other questions; only a small segment of the questions was used in Papers I and II. Prior to the implementation of the project in 1989, a pilot test of the questionnaires was conducted. In the first instance, 15 laypersons filled in the questionnaire and were interviewed as to whether the questions were comprehensible to laypersons. Subsequently, 200 expecting mothers filled in the questionnaires and were then interviewed and also asked whether the questions were comprehensible, answerable, and could be interpreted as offensive, and whether the time needed to answer the questions was acceptable.

Paper IIIThe primary aim of the Smoke-free Newborn study was to examine whether multimodal interventions for pregnant smokers could improve quitting rates during pregnancy (135). Furthermore the aim was to reduce, among all pregnant women, the proportion of pregnant women who were exposed to environmental tobacco smoking. All pregnant smokers in the intervention group were offered a smoking cessation program (135) and all pregnant smokers and non-smokers

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received oral and written information about the harmful effect of exposure to environmental tobacco smoke during pregnancy. The questionnaire was self-completed at 12-18 weeks of gestation and elicits details about socio-demographic characteristics, obstetric history, and lifestyle factors, including information about smoking status and leisure time physical activity before pregnancy.

As in Papers I-II the questions on physical activity in leisure time were developed and evaluated by Saltin (131;132). In the questionnaire, the women were asked: When you look back on the year before your pregnancy, which would you say is the most appropriate description of your leisure activities? Reading, watching television, or pursuing some other sedentary pursuits; walking, cycling, or other light exercise at least four hours a week (include Sunday walks, light gardening, and cycling/walking to work);sports or heavy gardening at least four hours a week; hard training and competitive sports regularly and several times a week. Altogether 55 women engaged in competitive sports, and therefore those engaging in competitive sports and moderate-to heavy physical activity respectively were analysed as one group. Leisure time activity was thus classified as: sedentary, light, moderate-to-heavy. The questionnaires were pilot tested in 50 pregnant women.

Information about the diagnosis pre-eclampsia was gathered from a database in the Obstetrical Department. Pre-eclampsia was defined as the presence of gestational hypertension >140/90 mmHg on at least two occasions, and proteinuria (>2+) on an urinary dip-stick or >300 mg/24h, occurring after 20 weeks gestation (136).

Paper IVThe questionnaires elicit details about infertility treatment, socio-demographic characteristics, leisure time physical activity, and smoking status during pregnancy.In the questionnaire the women were asked: When you look back on the year before your pregnancy, which would you say is the most appropriate description of your activities? Reading, watching television, or pursuing some other sedentary activities; walking, cycling, or other light exercise at least four hours a week (include Sunday walks, light gardening, and cycling/walking to work); sports or heavy gardening at least four hours a week;. hard training and competitive sports regularly and several times a week.Furthermore, the women were asked about their level of activity in the first, second, and third trimesters. Hence, leisure time activity was classified as: sedentary, light, moderate-to-heavy, or competitive sports. As in Papers I-II the questions on physical activity in leisure time were developed and evaluated by Saltin (131;132).

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The questionnaires contained numerous questions; only a small segment of the questions was used in Papers IV. Before project implementation, a pilot test of the questionnaires was conducted. In the pilot test, 86 expecting mothers filled in the questionnaire in week 37 of gestation, a test re-test was undertaken of nine women, 18 pregnant women responded to the questionnaire in writing as well as verbally in a telephone interview, 51 expecting mothers were interviewed about potential emotional impacts in connection with completion of the questionnaire, and 42 were interviewed about some questions relating to the upcoming delivery (137).

Paper VPaper V aimed to collect knowledge about women’s own experiences and perceptions of their physical activity during pregnancy. Information about these aspects of life can be gathered through interviews, whereas questionnaires are of less value in this connection (138). Thus, in this thesis the semi-structured qualitative research interview was chosen as method for Paper V (139). Individual interviews were carried out according to Kvale (139), and all performed by the author of this thesis. A thematic interview guide with open-ended questions was used. The guide has been tested in a pilot study, but the interviews were not included in the results. Themes in the guide were; “leisure time physical activity before pregnancy”, “motivations for leisure time physical activity before pregnancy”, ”what happened when you became pregnant”, and “motives for exercising-leisure time physical activity during pregnancy”. Only data on physical activity during pregnancy were reported in Paper V. The interviews were carried out in the woman´s home, at her place of work, or in the research department at Rigshospitalet, depending on her preference. A secretary transcribed the interviews verbatim, also noting non-verbal expressions like laughter and silences. The duration of the interviews ranged from 15 to 50 minutes. The time gap between delivery of the first baby and the interviews was 3-4 years. All interviews were conducted in the period February – July 2008.

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STATISTICAL ANALYSISPaper IInitial analyses of random distribution between maternal characteristics, obstetric history, and social and life style factors and preterm delivery were based on the Chi-square test and test for trend. Confounders were selected a priori. We chose to include parity, maternal age, years at school, maternal pre-pregnancy BMI, and smoking status during pregnancy as confounding factors in the logistic multivariate analysis between exposure and preterm birth.

Paper IIThe Chi-square test and the ANOVA test were used for initial analyses between maternal characteristics, obstetric history, and social and life style factors. A priori, pre-pregnancy BMI, parity, smoking during pregnancy, years at school and gestational age were chosen as confounders. We also included gestational age in all multivariate analyses as a covariate. Linear regression analysis was used to examine the association between physical exercise (0, 1-2, 3+ hours of sports weekly), types of sports, or leisure time physical activity (sedentary, light, moderate-to-heavy) and birth weight measured as a continuous variable. Logistic regression analysis was used to examine the association between physical exercise (0, 1-2, 3+ hours of sports weekly) or leisure time physical activity (sedentary, light, moderate-to-heavy), and low birth weight (<2500g vs. higher), and high birth weight (>4500 vs. lower). Paper IIIInitial Chi-square testing for independence within contingency tables was used. The associations between the level of leisure time physical activities (sedentary, light, moderate-to-heavy) and the risk of pre-eclampsia were analysed in multivariate logistic regression analysis. A priori, pre-pregnancy BMI, parity, and smoking status during pregnancy were chosen as confounders. As pre-pregnancy BMI could influence insulin resistance and thus the association between leisure time physical activity and risk of pre-eclampsia, stratified logistic regression analyses were used to assess these potentially modifying effects.

In paper I-III all tests were two-tailed, and a significance level less than 0.05 was chosen. The results of the logistic regression analysis were shown as odds ratio (OR), and 95% confidence intervals (CI) were applied.

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Paper IVChi-square testing for independence within contingency tables was used. The pre-pregnancy level of leisure time physical activity and the level of leisure time physical activity in first trimester were tested in relation maternal characteristics, obstetric history, social factors, health and life style characteristics. All tests were two-tailed, and a significance level less than 0.05 was chosen.

Content Analysis Paper VThe topic of nulliparous women’s experiences and views of leisure time physical activity during pregnancy is scantly described in other studies. As no predetermined theoretical framework structure existed on this topic, an inductive analysis approach was relevant(140). The transcribed interviews in Paper V were analysed by qualitative content analysis, which is a text or word analysis (141;142). Before World War II, content analysis had its roots in journalism and was used to analyse documents and to count words and phrases, and it has to expand to include interpretations (142). Burnard described that reading textual data involves interpretation (142). In Paper V Burnard´s 14-step analytical method (141) was used and is described in detail.

In qualitative research it is especially important to deal with one’s own pre-understanding of the subject, as everyone is affected by their pre-understanding (138). If researchers are not clear about this, it will affect their research work. Pre-understanding will mean that questions will be asked in the way that the researchers expect them to be answered. The risk is then that they will achieve results that only will reflect their own pre-understanding (138). Dahlberg writes(138) that through openness, one can deal with pre-understanding. If one possesses this openness about one’s own biases, one will be open to other aspects of the subject. Before starting this qualitative study the author of the thesis described her pre-understanding and discussed it with the main supervisor AKS and supervisor KP. The author´s pre-understanding was shaped by clinical work, as a midwife, research on pregnancy and leisure time physical activity, and her own pregnancies. The supervisors AKD, and KP all have experience from their clinical work as midwife and pediatric nurse respectively, as well as from their own pregnancies. The supervisor PD has experiences from his clinical work and his research on pregnancy and leisure time physical activity. Throughout the entire data collection period the author of the thesis (HKH) intentionally ”bridled” her pre-understanding as described by Dahlberg (138) in order that no particular opinions or aspects would be unconsciously emphasized. By “bridling” her pre-understanding she was aware of promoting “more” i.e. nuances, variations, but not “too much” which could be related to her own pre-understanding (138).

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ETHICAL CONSIDERATIONSPapers I and II.The research protocol was approved by the Regional Committee for Ethics and Science (No. 1991/2060) and the Danish Data Protection Agency. Informed consent was obtained from all participants.

Paper III. Study approval was obtained from the local research ethics committee (Journal no. KF-O2-084/95) and the Danish Data Protection Agency (http://www.datatilsynet.dk/eng/index.html). Informed consent was obtained from all participants. It was carefully explained to the participants that participation was voluntary and that they could withdraw from the study at any time with no implications for their further treatment. In addition, care was taken to ensure that in the event of a miscarriage or stillbirth, women were not pressed for an answer to the questionnaire.

Paper IV. The Danish Dystocia Study was conducted in accordance with the ethical principles of the Declaration of Helsinki (http://www.mta.net/e/policy/pdf/17c.pdf). As the study did not involve invasive procedures or interference with normal clinical treatment, in accordance with Danish law no approval by the Ethical Committee was needed. Permission was given by the Danish Data Protection Agency to set up a database (j.nr.2004-41-3995). Confidentiality during the collection of data and in the data analysis phase was ensured by following Good Epidemiological Practice (http://ieaweb.org) and observing the rules laid down by the Danish Data Protection Agency). Informed consent was obtained from all participants, after which they were informed that they could withdraw from the project without giving explanation and without consequences for their further care and treatment during pregnancy, delivery and the postpartum period

Paper V.Pregnant women received a letter by mail in which they were informed about the content and purpose of the qualitative study. This was followed up by a telephone call (one woman called in herself) in which they were invited to take part in the study. Written information was provided before the telephone call to ensure that the women had the opportunity to consider participation. They were told that participation was voluntary and that they could withdraw from the project at any time. Each woman was allowed to choose where and when she would be interviewed. During the interview she was told that she did not need to answer any

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question if she was reluctant to do this if during the interview the interviewer the author that the woman needed to talk about a subject not related to the topic, she allowed her to do so.

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RESULTS

Leisure time physical activity and preterm deliveryPaper I examined the associations between sports and leisure time physical activity early in the second trimester and preterm delivery in 5749 pregnant women; 3.7% were delivered preterm. Initially the proportion of preterm deliveries was described in relation to maternal characteristics, obstetric history, social factors, and lifestyle factors (Table I, Paper I).

Pregnant women practicing light leisure time physical activity had a 24% non-significantly reduced risk of preterm delivery and women practicing moderate-to-heavy leisure time physical activity had a 66% significantly, adjusted reduced risk of preterm delivery than had sedentary women (Table II, Paper I), see Figure 2.

Figure 2. Risk of preterm

delivery in relation to leisure time physical activity in early second trimester (n=5749). Adjusted for maternal age, years at school, pre-pregnancy BMI, and maternal smoking during pregnancy.

In total 26% (1,514 women) was engaged in a sport of some kind, whereas 74% (4235 women) was not. Pregnant women practicing a sport had an adjusted risk of preterm delivery (OR=0.77, 95% CI:0.53-1.11) compared to women not practicing sports. No association between hours of sports per day and risk of preterm delivery was found. Women engaged in more than one sports activity had a 91%

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significantly adjusted reduced risk of preterm delivery (OR=0.09, 95% CI:0.01-0.66). (Table II, Paper I).

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Sports and leisure time physical activity during pregnancy and mean birth weight.Paper II investigates the association between sports and other leisure time physical activities early in the second trimester or early third trimester and birth weight in 4,558 women. Only participants delivered after 37 weeks of gestation were included. Birth weight was categorized in the following outcomes: continuous variable, low birth weight (<2500g vs. higher), or high birth weight (>4500g vs. lower). Initially the socio-demographic factors, obstetric history, and lifestyle characteristics weight were described in relation to birth weight (Table I, Paper II). The mean birth weight depended on pre-pregnancy BMI, parity, cigarettes smoked per day, and gestational age. During early second trimester, 18% (828 women) were engaged in sports 1-2 hours weekly and 9% (422 women) three hours or more weekly. During early third trimester, 13% (597 women) were engaged in sports 1-2 hours weekly and 6% (289 women) three hours or more weekly. The multivariate analysis found that pregnant women who practiced sports gave birth to infants with the same adjusted mean birth weight as less active women (Table II, Paper II), see Figure 3.

Figure 3. Adjusted birth weight in relation to hours of sports weekly (n=4558). Multivariate linear regression analyses adjusted for parity, pre-pregnancy BMI, maternal smoking during pregnancy, years at school and gestational age

When the data were analysed according to the type of sports practiced the results were essentially the same (data not given). Early in the second trimester, 41% (n=1894) were pursuing sedentary activities, 52% (n=2357) light activities, and 7% (n=307) moderate-to-heavy activities. Early in the third trimester, 44%

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(n=1998) pursued sedentary activities, 52% (n=2384) light activities and 4% (n=176) moderate-to-heavy activities. Women engaged in light leisure time physical activity and women engaged in moderate-to-heavy leisure time physical activity early in the second or third trimester gave birth to infants with a similar adjusted mean birth weight as women with sedentary activities did.

Sports and leisure time physical activity and delivery of infants with low birth weight or high birth weight, respectively.Women practicing sports 1-2 hours weekly or 3 hours weekly early in the second or third trimester had a similar adjusted risk of infants being born with a birth weight <2500g vs. higher and >4500g vs. lower after 37 weeks of gestation as had women not engaged in sports (Figure 4).

Figure 4. Adjusted risk (OR) of an infant with a birth weight <2500g vs. higher and >4500g vs. lover in relation to sports per week early in the second or third trimester (n=4458). Adjusted for parity, maternal pre-pregnancy BMI, maternal smoking during pregnancy, years at school, and gestational age.

Women practicing light or moderate-to-heavy physical activity early in the second or third trimester had a similar adjusted risk of infants being born with a birth weight <2500g vs. higher and >4500g vs. lower after 37 weeks of gestation as had women with sedentary activities.

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Leisure time physical activity before pregnancy and pre-eclampsia.Paper III reported the association between leisure time physical activity the year before pregnancy and pre-eclampsia in a population of 2793 women. In total 14% (381 women) pursued sedentary activities, 70% (1959 women) practiced light physical activity, and 16% (453 women) practiced moderate-to-heavy activity. Pre-eclampsia was diagnosed in 4% of the women (112 women). The proportion of those with pre-eclampsia depended on the level of pre-pregnancy BMI and parity (Table I, Paper III). The highest frequency of pre-eclampsia (12.6%) was found in obese women and in nulliparous women (5.8%). The proportion of pre-eclampsia was 4.2%, 4.2%, and 3.1%, respectively in women engaged in sedentary, light, and moderate-to-heavy leisure time physical activity. However, women who practiced moderate-to-heavy activity had an adjusted, non-significantly reduced risk of pre-eclampsia compared to sedentary women. The respective risk was (OR=0.6, 95% CI:0.3-1.4). The results remained similar whether or not BMI was included in the analysis (Paper III; Table II). In the multivariate analyses stratified for BMI, those practicing light leisure time physical activities and those practicing moderate-to-heavy physical activities had an unchanged risk of pre-eclampsia compared to sedentary women.

Patterns of leisure time physical activity before and during pregnancyIn the year before pregnancy 4% (192 women) were practicing competitive sports, 25% (1178 women) moderate-to-heavy activities, 68% (3119 women) light activities, and 5% (261 women) sedentary activities. During pregnancy, a lower proportion of women were practicing competitive sports and moderate-to-heavy leisure time activities whereas the proportion of women performing light activities was slightly higher and the proportion of sedentary women much higher than before pregnancy (Figure 5).

Level of leisure time physical activity during pregnancy related to pre-pregnancy leisure time physical activity.

Pregnant women engaged in competitive sports or moderate-to-heavy activities before pregnancy reduced the level of their activities by around 50% in first trimester. In the group engaged in light activities before pregnancy most of the women continued at this level, and 70% were still practicing light activities in the third trimester. In the group of women who were mostly sedentary before

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pregnancy, the majority continued at that level, but some changed during pregnancy and 25% practiced light leisure time physical activity in third trimester. Changes in leisure time physical activity from prior to pregnancy through the first trimester were examined in relation to socio-demographic characteristics, fertility history, and lifestyle factors. In the group of women engaged in moderate-to-heavy leisure time physical activities before pregnancy, women who were overweight (BMI>25 kg/m2), and women treated for infertility more often reduced this level in the first trimester compared to women of normal weight and women who had not been treated for infertility. In the group of women engaged in light activities before pregnancy, non-smokers more often reduced their physical activity level during the first trimester than did smokers, and women treated for infertility also more often reduced their activities than others.

Figure 5. Proportion of women with different levels of leisure time physical activity before and during pregnancy (n=4718).

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Experiences and views of leisure time physical activity during pregnancy among nulliparous women.The four main categories developed to elucidate the experiences and views of nulliparous women of leisure-time physical activity were: Physical activity as a lifestyle, Body awareness, Carefulness, and Sense of benefit. All nineteen interviewed respondents were engaged in moderate-to-heavy leisure-time physical activity prior to pregnancy.

Physical activity as a lifestyle

Sub-categories developed in this category were: Habit and Desire to continue. The women experienced that they remained physically active out of sheer habit, many without giving it a thought. Others kept up their leisure time physical activity by choice, as they enjoyed it, wanted to stay in shape, and believed it would benefit the pregnancy and delivery. Apart from this, they could not do without the sense of well-being afforded by exercise, were acquainted with other women who had been physically active during pregnancy, and felt that they could also be so. Many perceived that they could continue to be physically active but often to a lesser degree and less intensively. Few women increased the level of exercise.

”I was quite determined that my pregnancy should not stop me from being physically active… because I have always enjoyed exercising and I was sure that everything would go much better if I was physically active (I:5).

”I felt it was just part of my daily routine and part of my life, so I simply went on with it (I:4).

Body awareness

Kategories of this category were: Pregnancy-related discomfort and Having a complicated pregnancy and Growing body. Physiological changes during pregnancy impacted physical activity. Pregnancy-related complications such as the need to urinate frequently during aerobics or pressure on the bladder while jogging were also reasons for ceasing to perform various sports. Those women who experienced bad health during pregnancy also found themselves forced to suspend their activities, some remaining inactive for the remainder of the pregnancy, others being inactive for a time and then taking up less intensive activities. The growing

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abdomen during pregnancy caused women to stop or change their activities. Women jogged more slowly and shorter distances, and could not cycle on bicycles with forward-leaning handlebars. The women did find that they could continue to cycle if they had a bicycle with upright handlebars. Ordinary pregnancy-related issues such as fatigue and nausea also caused the expecting mothers to change their physical activities during pregnancy. All the women also sensed that the growing body forced them to find new activities or to change activities if they were to continue to be active. The women viewed the new sports activities and adjustment of activities as beneficial for the growing body.

”just before I discovered that I was pregnant I was at a spinning class with my husband where at the end of the class I felt so awful, worse than I have ever experienced… I simply couldn’t go on” (I:1).

”so I had problems because I had to lean forward so much on my bike, so at the end when I got other handlebars put on and later exchanged bikes with a friend who had one of these old-fashioned bikes, then I could sit upright so I could bike” (I:11).

Carefulness

Subcategories of this main category were: Feelings of worry and Balancing worry and sense of security, the latter with the sub-sub-categories Security from within and Security from without. All expecting mothers, some more than others, experienced worry about being physically active during the pregnancy and this influenced their activity level. Most worried were those who had previously miscarried or were pregnant after infertility treatment, fearing miscarriage if they undertook excessive physical activity, and they were therefore cautious.

”but before I had Peter I had a miscarriage, so I had sort of a period where I didn’t dare dance as wildly again while I was expecting Peter, so I was especially careful (I:13).

Several nulliparous decided against jogging and strength training because they felt these were too strenuous and were afraid they would miscarry. Despite their concern however, most chose to continue to be active, finding a certain sense of security in doing so. Some found a sense of security from within in being able to judge for themselves what they could or could not manage, others switched to alternative activities and felt secure in this. Others were given an exercise program by physiotherapists or instructors, which they found reassuring. Finally, some

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participated in prenatal classes, which they found, gave a sense of security, as the trainers were knowledgeable of what pregnant women can tolerate.

”no, well I did sort of feel that I would be able to sense from my body if something was wrong” (I:6).

”in the training center that had exercise sessions for pregnant women, it was as if it was within the pregnancy limits, there they knew how much you could take…I mean they were aware that they were dealing with pregnant women” (I:3.)

Sense of benefit

The category involved the two subcategories Feelings of happiness and Physical well-being. Many pregnant women felt that physical activity during pregnancy gave them much enjoyment, energy, and psychological well being. The expecting mothers also felt that physical activity brought them physical well being and reduced pregnancy-related discomfort such as edemas and back pain. Swimming was described as pleasant. When pregnant women swim they feel weightless and unencumbered while still exercising their bodies.

”just being active, it gave me energy and made me happy, it made me wildly happy” (I:4).

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DISCUSSION

Methodological issuesIn this thesis both quantitative and qualitative methods are used as quantitative and qualitative studies in research may supplement each other (143). This is illustrated in Papers IV and V. The level of physical activity during pregnancy in a large population is described in Paper IV, and a small proportion of these women described their own views of and experiences with their level of leisure time physical activity in Paper V.

Quantitative studies

Data sourcesRather than setting up a new cohort it was decided (the author HKH) to use databases from three pre-existing studies for this thesis. There were several advantages using these three databases. They are comprehensive and have high response rates, and the questions that were asked in these three pre-existing studies regarding sports and physical activity were the same as would have been asked if a new cohort was set up for this thesis. It shows respect for the participants in these studies to utilize all the data on subjects for which they have provided information. It also avoids inconveniencing other pregnant women unduly by asking the same questions about sports and physical activity. In addition, it is both more economical and more expedient to use pre-existing data.

Data sources, papers I and II Papers I and II are based on data from “The Aarhus Birth Cohort 1989-91”. It may be relevant to discuss whether data gathered more than 20 years ago have any scientific value today. However the purpose of our study was to investigate whether leisure time activity and sports during pregnancy are associated with preterm delivery. Regardless of present-day exercise and activities, our findings may add to the body of knowledge and understanding of this association. In fact, one study has shown that pregnant women’s level of leisure time physical activity has remained largely unchanged over the last twenty years (127).

Data sources, Paper III In Paper III we took the liberty of using data from the “Smoke-free Newborn Study”, an intervention study investigating the association between exposure (leisure time physical activity in the year prior to pregnancy) and outcome (pre-eclampsia). Maternal smoking was associated with a decreased risk of pre-

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eclampsia and theoretically our results might have been influenced by the smoking cessation intervention, which was the primary aim of the Smoke-free Newborn Study. To eliminate this potential bias, smoking status was classified in the multivariate analyses as: smokers, quitters in pregnancy, and non-smokers .

Furthermore we tested in the multivariate analyses if the association between leisure time physical activity and pre-eclampsia was influenced by the women’s assignment in the intervention study to either the intervention group or the control group. This did not change the risk estimates considerably (data not shown). A weakness in using the database from the Smoke-free Newborn Study in Paper III is that there are only a few women performing moderate-to heavy leisure time physical activity with a BMI >25 kg/m2.. Thus, the risk estimates were determined with broad confidence intervals.

Internal validityIt is important to avoid systematic errors (bias) which can lead to an incorrect description of the associations between leisure time physical activity or sports and preterm delivery, birth weight and pre-eclampsia. Internal validity is about avoiding these mistakes, and concerns selection bias, information bias and confounding (144).

Selection bias, Papers I and IIAs Papers I and II are based on data collected prospectively, the risk of selection bias is minimized, the subjects having been included before the outcome became known (144). However, 20% of all those responding to questionnaire 1 did not respond to questionnaire 2 and 13% gave incomplete information on sports and leisure time physical activity. An earlier study (42) of the same cohort showed a higher proportion of preterm births among those who did not respond to questionnaire 2 than among those who did respond. There might be a potential selection bias if non-respondents both were more inactive and had more premature births. This information was not available, but if it was the case, it would be a bias that would underestimate the result.

Selection bias, Paper III Data is from an intervention study “Smoke-free Newborn Study” and in this study the allocation was based on birth dates known to the midwives recruiting the subjects. It may have influenced the recruitment of participants and thereby the study population if some types of pregnant women were more strongly encouraged to participate than others, and imply that the study was affected by selection bias. There was however, no difference between the control group and the intervention group in relation to known background variables, smoking habits and lifestyle factors during the first antenatal visit (135). This would seem to minimize the risk of selection bias in Paper III.

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Information bias (exposure), Papers I-IV The exposure variable in Papers I-V is leisure time physical activity and in Paper IV it is also sports. Assessment of leisure time physical activity relied on self-reporting, which has the advantage, compared to objective measures, that it is easy to administer at a low costs, but it entails a risk of misclassification (145).

The question on leisure time physical activity was validated. One review (146)stresses the importance of using validated questionnaires on physical activity among pregnant women, but indicates that few studies have done so, which is a strength in Papers I-IV. The questionnaire about leisure time physical activity contains a measurement of intensity which is missing in other studies (50;51). As perception of intensity may depend on age, gender, fitness and duration, misclassification may occur even though relative and absolute intensity are very similar in a homogeneous sample, which may be the case in a group of pregnant women (145). However, we cannot rule out the bias that pregnant women who practiced sedentary activities or competitive sports prior to pregnancy have varying perceptions of light leisure time physical activity during pregnancy because their pre-pregnancy fitness levels were different. Saltin´s questions on leisure time physical activity were developed for men in the 1960’s (22), which perhaps explains why they include gardening but not housework or caregiving. This could be a weakness, as household/caregiving is a large part of the overall physical activity of pregnant women (124;147).

Information about exposure was collected prospectively at both 16 and 30 gestational weeks increasing validity. In Papers III and IV women were asked about their leisure time physical activities in the year before their pregnancy. This could entail a risk of recall bias, but studies have found that women recall past years’ total physical activity and especially vigorous activity with high reliability (148).

There were no national recommendations on physical activity during pregnancy at the time of data collection for Papers I and II. There is therefore no reason to believe that the pregnant women under- or overestimated their activities with a view to comply with guidelines, as no guidelines were in place. In Paper IV it is relevant to note however, the risk that expecting mothers are overestimating the level of their physical activity during pregnancy. Shortly before data was collected for Paper IV the Danish National Board of Health issued recommendations that pregnant women take ½ hour of moderate exercise daily. Thus, some pregnant women may have wished to indicate that they were following these recommendations and thus have overstated their activities. This mechanism has been observed in studies on smoking (149). Also, although the questions on activity in Paper IV deal with leisure time physical activity, “leisure

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time” is not specifically defined. Thus, there was risk that the women included their work-related activities such as dance or physical education instructors. Information bias (outcome), Papers I-IIIIn Paper I the outcome is delivery before 37 completed gestational week (22) and gestational age is estimated in three ways: 1) ultrasound measurement before week 21, 2) date of latest certain menstruation, 3) estimation by the midwife on the basis of uncertain latest menstruation or an ultrasound measurement after 21 weeks.There is no reason to believe that uncertain measurements of gestational age based on ultrasound scanning before gestational week 21 or the date of last certain menstruation differ between exposed and non-exposed. It may be relevant however, to assess whether estimations of gestational age made by the midwife based on uncertain last menstruation or an ultrasound measurement after 21 weeks differ between exposed and non-exposed pregnant women. The latter is relevant, as these women may belong to a special group of expecting mothers who, for example, ascertain the pregnancy late. During statistical analyses we have assessed whether the risk estimate between exposure and preterm delivery changes when excluding the group with uncertain latest menstruation and late ultrasound. The risk estimate did not change, thus there is no indication of a differential misclassification, although there may be a non-differential misclassification. A non-differential misclassification does not affect the risk estimate or draw the result toward the null hypothesis.

In Paper II birth weight is outcome. The birth weight of the child is measured by midwives with no knowledge of the woman’s exposure to sports/leisure time physical activity. Although there may be uncertainty when measuring the weight of the child at birth, there is no reason to believe that this uncertainty is associated with exposures, thus minimizing the risk of differential misclassification. Birth weight of <2500 g is taken as an outcome; this may be subject to criticism, as a birth weight of <2500 g may not necessarily be an expression of reduced growth but simply reflects that the parents and thus the infant are genetically small (150). This criticism is valid. A second point of criticism is that infants with a birth weight <2500 g might be born preterm and their growth as such is therefore not reduced. We have adjusted for this by only including women who give birth after 37 gestational weeks and by correcting for gestational age. In Paper III pre-eclampisa is outcome. Pre-eclampsia was described on the basis of the mother’s blood pressure and measurement of protein in the urine. These measurements were objective and taken by either midwives or nurses. There will always be uncertainty in these measurements but there is no reason to believe that the measurements were different for the various exposure groups.

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ConfoundingConfounding is present if an additional variable that is responsible for some of observed association is not equally distributed in the exposure and the non-exposure groups, respectively (144). To avoid confounding we had adjusted, in Papers I-III, for potential confounders in multivariate analyses. Potential confounders were chosen a priori, as other studies have shown that these variables are risk factors for the outcome.

Although a statistical association was seen between previous preterm deliveries and preterm delivery in the present pregnancy we did not include this factor as a potential confounder as this elevated risk may be a part of the effect as the same exposure in a previous pregnancy (151).

In Paper III it was difficult to determine whether pre-pregnancy BMI was an intermediate variable or a confounder, for which reason analyses have been conducted with and without adjustment for BMI.

If the women discussed in Paper I were inactive because they showed incipient signs of pre-term birth, there will be a risk of confounding by indication (144). This problem was overcome in Paper I by only investigating the association between sports/leisure time physical activity at 16 gestational weeks and preterm delivery. At this early time in pregnancy it is presumed that there was no symptoms present, which may indicate later preterm birth. In Papers I and II we chose to exclude pregnant women who suffered from a chronic disease in order to avoid the effect that it was healthy women who were active.

As we had no information on diseases in the expecting mothers prior to pregnancy in Paper III it was not possible to exclude these, which may be a limitation, as there is a risk of confounding by indication.

When adjustment for confounding was made in multivariate analyses by stratification, there is a risk of residual confounding if the confounder variables’ strata were too broad (144). In Papers I and II this was avoidable, as the population was large and many strata could be established.

In Paper III the proportion of women with pre-eclampsia increased with increasing BMI; the proportion of pre-eclampia was 1.9% in women with a BMI <18.5 kg/m2and 12.6% in women with >30 kg/m2. If, in the multivariate analysis, it is opted to use BMI as a confounder where BMI is dichotomised (<25 kg/m 2; >25 kg/m2) there will be a potential risk of residual confounding, as this division into strata will not reflect the same differences in the frequency of pre-eclampsia in the

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various BMI groups. To avoid residual confounding, risk estimates for both narrow and broad strata were performed, and no relevant significance was seen.

External validity (Papers I-IV)Papers I and II are population-based studies and data have been collected in a single city of 250,000 inhabitants where there was only one birth center. Women from all social strata therefore gave birth at this center, but mothers with multiple pregnancies and women not speaking Danish were excluded from analyses. The results may therefore be generalized to a broad range of Danish females having a singleton pregnancy.

Paper III is likewise a population-based study from a hospital in Copenhagen where local women with medium and low incomes gave birth. Women who did not speak Danish, women pregnant with multiple pregnancies and women with alcohol or substance abuse were not included in the population and data of this paper can therefore not be applied to these women. The study population in Paper IV involved only nulliparous with no planned induction or caesarean section, thus the results may be generalized only to this group.

Qualitative study

Trustworthiness is an important concept in qualitative studies, comprising concepts such as credibility, dependability and transferability, which parallel the concepts validity, reliability and applicability in the quantitative tradition. The qualitative study will be discussed in relation to these three concepts.

Credibility (the true value) presumes, among other things, that the selection of participants with differing experiences affords a better opportunity to elucidate the research subject from various aspects (152). We have attempted this in our study by undertaking purposive sampling for participants with a wide variation in age, pre-pregnancy BMI, chronic diseases, education and lifestyle factors during pregnancy. A discussion within the research group of codes and sub-categories was also a means to enhance credibility. In the entire research process the author (HKH) bridled her pre-understanding (153). In this way, space was afforded for a much more varied and nuanced description than simple reflections on the part of HKH of her own pre-understanding. A possible bias is that women were interviewed 3-4 years after the birth of their first child. Simkin. reported however, that women clearly remember their first birth after several years (103). The level of detail in the women’s descriptions in this study likewise indicated that women clearly recall their first pregnancy.

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Dependability relates to the extent to which data change over time (152), and whether the analytical decisions of authors change over time. We have attempted to accommodate this by consistently asking all participants the same introductory question and through discussions among the authors of coding, sub-categories and categories.

Transferability concerns transposition, and Graneheim et Lundman (152) is of the opinion that it is at the discretion of the reader whether the data are transferable. In this context, transferability is enhanced by the fact that participants have been selected with a view to a certain variation and are described. There is no reason to believe that the same findings would not appear in a group of women having the same characteristics.

Discussion of findings Paper I showed that women engaged in moderate-to-heavy leisure time physical activity more than 3 hours a week had a significantly reduced risk of preterm birth compared to sedentary women, whereas this was not the case for pregnant women with light leisure time activities more than 3 hours weekly. Other studies showed similar risk reduction in women performing sports activities compared to inactive women (49;51;53). The finding that the intensity of leisure time physical activity influenced the risk of preterm delivery is a new observation since association between intensity of physical activity and preterm delivery has not previously been studied in larger studies. Preterm delivery has been associated with an increased low-grade inflammation as illustrated by increased proinflammatory cytokine levels (e.g. tumor necrosis factor-α (TNF-α) (154;155). Exercise seems to reduce the concentrations of TNF-α in pregnant and non-pregnant persons (156;157). Thus as mentioned in Paper I, a hypothetical biological explanation for the lower prevalence of preterm delivery in physically active women might be a reduced low-grade inflammation in comparison to physically inactive pregnant women (154-157). In recommendations on physical activity for non-pregnant women, the most recent American guidelines point out that moderately intensive physical activity (described as brisk walking) should be supplemented by more vigorous physical activities to attain the best effect on health (2). American and Danish recommendations recommended moderate-intensive leisure time physical activity (brisk walking) to healthy women with normal pregnancy. In relation to paper I it could be relevant to consider, if moderate intensive psychical activity during pregnancy supplemented by more vigorous psychical activity also could attain the best effect of health in pregnancy.

Paper II showed that pregnant women engaged in sports 1-2 times per week or 3 hours per week, or those having 3 hours per week of light or moderate-to-heavy

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levels of recreational activity, gave birth to infants with similar birth weights as physically inactive pregnant women.This is in line with results from many other studies about leisure time physical activity (56;73;76;76;79;81;84;92) but in contrast to earlier, smaller prospective studies in selected populations (74;75;82). A recent study from a military base showed that the women who exercised most gave birth to infants with a reduced birth weight, on average of 87 g as compared to children born of women who exercised least (77). Exercise also encompassed mandatory exercise during part of the pregnancy; it is therefore not possible to make a direct comparison with the results of Paper II, where all physical activity was undertaken during leisure time (80;89;91). The risk of delivering an infant with a birth weight of <2500g was similar, regardless of whether the pregnant women were engaged in sports, performed light or moderate-to-heavy leisure time physical activity or were sedentary in paper II. Other studies found either a similar result (76), or a higher risk of delivering an infant with a low birth weight in exercising women compared to non exercising women (55;88). It is difficult to compare results from Paper II with results from these studies (55;88), as some infants with a low birth weight in these papers may have been born preterm (150) which was not the case in Paper II, as women with preterm deliveries were excluded.

To the authors knowledge there are no other studies that have investigated the association between sports or leisure time physical activity and infants with birth weights >4500g. This should be the subject of a future examination.

Previous studies described that the distribution of blood flow to the skin and working muscles during exercise might reduce uterine blood flow (70),and thereby growth could theoretically be compromised. In Paper II neither sports more than 3 hours weekly nor moderate-to-heavy leisure time physical activity were associated with a diminished birth weight.

In Paper III an unchanged risk of pre-eclampsia was found in women with more than four hours per week of light or moderate-to-heavy pre-pregnancy leisure time physical activity compared with women who were sedentary before pregnancy. This is in accordance with results found in some studies (99;112) but contrasts with other studies which found that leisure time physical activity before pregnancy protected against pre-eclampsia (108;109;113). Rudra et al. (113) found that the inverse association between perceived exertion during recreational physical activity before pregnancy and risk of pre-eclampsia was more apparent among women with a BMI >25 kg/m2 than among women with a BMI <25kg/m2. It is therefore somewhat surprising that this is not found in Paper III’s study, as it is biologically plausible that exercise may prevent pre-eclampsia in overweight

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women. Overweight women are characterized by insulin resistance, increased inflammatory system response and endothelia dysfunction (98), both risk factors or characteristics of pre-eclampsia (107). Exercise have a beneficial effect on these factors (3). In Paper III, among women with BMI >25 kg/m2 we found a tendency towards a lower risk of pre-eclampsia in those with the highest degree of physical activity during leisure time, but no significant associations. This non-significant result may be because the population was too small to identify a difference, or that there was none. This study does make a contribution however, by drawing attention to the possibility that physical activity prior to pregnancy may protect women with a BMI >25 kg/m2 against the development of pre-eclampsia. This is relevant, as the proportion of overweight individuals is increasing globally (158). A review article (102) showed that the risk of pre-eclampsia doubles with a BMI increase of 5-7 kg/m2 and therefore a future increase in the frequency of pre-eclampsia may be expected. As women with pre-eclampsia have a heightened risk of cardio-vascular diseases later in life,(97) it would be appropriate to encourage pregnant women who have experienced pre-eclampsia during pregnancy to generally increase their level of physical activity.

Paper IV indicated that those women whose lifestyle prior to pregnancy is sedentary constitute a vulnerable group, as apart from their inactivity they are often characterized by being smokers, overweight, lower educated and young mothers; other studies have made similar findings (114;123;127). It is relevant to focus on this group in the future.

Like other studies (15;114;116-118;120), Paper IV showed that expecting mothers reduced their level of recreational physical activity during pregnancy with regard to both amount and intensity. As all studies worldwide give this consistent description of reduction in intensity and amount, this would seem to indicate a universal, naturally inherent and inevitable phenomenon. Paper IV thus showed that the vast majority of women who prior to pregnancy engaged in competitive sports, or in moderate-to-heavy activity, reduced this level. This might be advantage with respect of the finding of a recent the Danish study showing that exercise and particularly high impact exercise during pregnancy was associated with an increased risk of miscarriage in the first half of pregnancy (159). Paper V outlined the experience of the women that it was best to discontinue activities such as jogging or strength training during pregnancy and indicated that pregnant women could sense whether their leisure time physical activity was too vigorous for them. Danish National recommendations for pregnant women prescribe (10) that women with high levels of leisure time physical activity prior to pregnancy can continue during pregnancy - as long as they otherwise are in good health. Descriptions from paper V indicated that women performing high levels of sports

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before pregnancy know when they could continue certain types of sports during pregnancy and when they could not.

In Paper V it was women’s experience that although they intended to continue to be active, they were so at a physically less intensive level due to psychological and the natural physiological barriers that arise in both late and early pregnancy, barriers that also are described in other studies (13;14;128). To this discussion however, Paper V adds the descriptions of pregnant women themselves of their experiences in overcoming these barriers so that they do not entirely cease to be physically active, but continued to be so at a less intensive level or in other activities. Some women managed to overcome these barriers themselves others need counseling e.g by midwives or physiotherapists.

In total, 25% of women with sedentary lifestyles before pregnancy performing light leisure time physical activity during pregnancy, maybe because pregnant women in general strive for at healthier lifestyle. It is positive that the majority of pregnant women in Paper V performed light leisure time physical activity during pregnancy as has many potential benefits (160-162).

Lastly, as studies among nonpregnant women have shown that reducing leisure time physical activity can influence the risk of some diseases it is relevant to consider if a sedentary lifestyle in one or more pregnancies can influence a woman’s long term health (17).

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CONCLUSIONSConclusions: In line with the majority of previous studies this thesis found a lower risk of preterm delivery in leisure time physically active women. A new finding was that this association was present in primarily women with the highest intensity level of leisure time physical activity. In contrast, no associations between leisure time physical activity and birth weight or risk of pre-eclampsia were shown. Finally several studies describe barriers faced by pregnant women against being physically active during pregnancy. Accordingly the present thesis indicates that leisure time physical activity level in general decreases gradually from before pregnancy to the first and third trimester in the majority of women. Using a qualitative approach a new finding was that women described their experiences in overcoming these barriers in order to continue to be psychically active during their pregnancyBased on this thesis no indications of disadvantageous effects of maternal leisure time physical activity for either mother of offspring were documented. In contrast psychological and physical benefits for the mother were indicated in the qualitative study, and for the offspring a potentially reduced risk of preterm delivery was outlined. Thus the thesis per se gives no indications for a change in the Danish National Board of Health’s guidelines about leisure time physical activity during pregnancy.

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FUTURE PERSPECTIVESThe findings and conclusions of this thesis identified new research questions about pregnancy and leisure time physical activity:

To validate Saltins question`on leisure time physical activity among pregnant women

To examine the relation between the level of leisure time physical activity during pregnancy and worries and psychological wellbeing, respectively.

To examine the association between the level of sports and leisure time physical activity during early third trimester and the physical well-being of the child at birth measured in Apgar score and ph-value.

In a randomized design to examine if an exercise intervention including swimming and bicycling could reduce the risk of pre-eclampsia in pre-pregnancy inactive overweight women.

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SUMMARY IN DANISH

Baggrund for afhandlingen:Regelmæssig fysisk aktivitet i fritiden hos ikke gravide er forbundet med psykisk og fysisk velbefindende og en reduceret risiko for mange sygdomme, herunder hjertekarsygdomme og type-2 sukkersyge. Man har derfor antaget, at fysisk aktivitet i fritiden under graviditet også har en positiv indvirkning på graviditetsforløbet. Men der har også været udtrykt bekymring for, om fysisk aktivitet i fritiden under en graviditet kunne være forbundet med risici som for tidlig fødsel eller for lille vækst af fosteret.

De nyeste amerikanske anbefalinger fra 2002 om graviditet og fysisk aktivitet i fritiden anbefalede gravide kvinder uden medicinske og obstetriske komplikationer at være fysisk aktive i fritiden (moderat intensitet 30 minutter per dag). Moderat intensitet er beskrevet som f.eks. rask gang. I 2004 kom Sundhedsstyrelsen med en dansk anbefaling til gravide, som var meget lig den amerikanske anbefaling, men som ydermere tilrådede raske kvinder med disposition til svangerskabsforgiftning eller graviditetssukkersyge at være fysisk aktive udover det anbefalede med hensyn til mængde og intensitet. Til trods for disse anbefalinger efterspurgte flere studier yderligere viden om sammenhæng mellem fysisk aktivitet i fritiden og barnets fødselsvægt, risikoen for en for tidlig fødsel og risikoen for at udvikle svangerskabsforgiftning. Hvis man skal målrette råd om fysisk aktivitet til gravide, er det vigtigt at have kendskab til gravide kvinders fysiske aktivitetsniveau før og under graviditet, samt til gravide kvinders egne oplevelser og erfaringer med at være fysisk aktiv under graviditeten. Specielt sidstnævnte er kun beskrevet i få studier.

Formål:De overordnede formål med afhandlingen var at undersøge sammenhænge mellem fysisk aktivitet i fritiden før og under graviditeten og risikoen for en for tidlig fødsel, påvirkning af barnets fødselsvægt og risikoen for at udvikle svangerskabsforgiftning. Endvidere blev kvindernes fysiske aktivitetsniveau før og under graviditeten beskrevet, og endelig blev deres oplevelser og syn på fysisk aktivitet under graviditeten belyst.Afhandlingen omfatter fem delarbejder. De fire første delarbejder er kvantitative studier og baseret på allerede eksisterende datakilder: Den Aarhusianske fødselskohorte (1989-91), Projekt Røgfri Nyfødt (1996-1997) og VeProjekt kohorten (2004-2005). Der var mellem 2750 og 5750 kvinder i hver studie population. Det femte delarbejde er et kvalitativt studium, og datakilden er interviews af 19 kvinder fra Veprojekt kohorten. I spørgeskemaerne blev de

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gravide kvinder spurgt om deltagelse i sportsaktiviteter og deres fysisk aktivitets niveau i fritiden. Sidstnævnte blev kategoriseret som: stillesiddende, let aktivitet (eks gå ture), moderat-tungt aktivitet (motionsidræt og hårdt have arbejde) og konkurrence idræt.

Afhandlingens fund:Delarbejde I viste, at gravide kvinder med et moderat-tungt fysisk aktivitetsniveau i fritiden (motionsidræt eller tungt havearbejde mere end 3 timer om ugen) havde en 66% nedsat risiko på for en for tidlig fødsel (mere end 3 uger før termin) i sammenligning med fysisk inaktive gravide, medens gravide kvinder med et let fysisk aktivitetsniveau ikke havde en nedsat risiko for en for tidlig fødsel. Gravide kvinder, der dyrkede mere end én type af sportsaktiviteter under graviditeten, havde en 91% nedsat risiko for en for tidlig fødsel i sammenligning med gravide kvinder, der ikke dyrkede sport.

Delarbejde II viste, at gravide kvinder, der dyrkede sport enten 1-2 timer per uge eller mere end 3 timer per uge, fødte børn med samme fødselsvægt som ikke sportsudøvende kvinder. De sportsaktive kvinder havde også samme risiko for at føde et barn med lav fødselsvægt (<2500 g) eller høj fødselsvægt (>4500g) som kvinder, der ikke dyrkede sport under graviditeten. Tilsvarende fund sås, når sammenhænge mellem gravides fysiske aktivitetsniveau i fritiden og barnets fødselsvægt blev undersøgt.

Delarbejde III viste, at det fysiske aktivitetsniveau i fritiden året forud for graviditeten ikke havde betydning for, om kvinderne udviklede svangerskabsforgiftning eller ej. Dette gjaldt for såvel normalvægtige som overvægtige kvinder.

Delarbejde IV viste, at forud for graviditeten var 4% aktive med konkurrenceidræt, 25% med motionsidræt eller tungt havearbejde mere end 4 timer per uge, 66% med lette fysiske aktiviteter (herunder gåture/cykle ture) mere end 4 timer per uge og 5% med stillesiddende aktiviteter. Under graviditeten dalede den fysiske aktivitet i fritiden generelt. Andelen af kvinder, der udførte konkurrenceidræt og motionsidræt/tungt havearbejde faldt markant, andelen af gravide med lette aktiviteter steg svagt, og andelen af gravide med stillesiddende aktiviteter steg fra 6% forud for graviditeten til 29% i sidste tredjedel af graviditeten. Gruppen af gravide, som var mest fysisk aktive før graviditeten fortsatte med at være fysisk aktive på et højere niveau under graviditeten end kvinder, der dyrkede fysisk aktivitet på et lavere niveau forud for graviditeten.

Delarbejde V belyste detaljerede oplevelser og syn på fysisk aktivitet i fritiden hos 19 kvinder, som skulle føde for første gang. Alle de 19 gravide havde et fysisk

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aktivitetsniveau i fritiden svarende til motionsidræt eller tungt havearbejde mere end 4 timer om ugen før graviditeten, mens deres aktivitetsniveauer under graviditeten varierede. Undersøgelsen viste at de gravide oplevede, at de fortsatte med fysiske aktivitet i fritiden under graviditeten, fordi det var en vane eller et stærkt ønske. Graviditetskomplikationer, graviditetsgener, den voksende gravide krop samt bekymring for om fysisk aktivitet under graviditeten var farligt, opleves som barrierer for at fortsætte den fysiske aktivitet under graviditeten. Skønt de gravide oplevede såvel fysiske som psykologiske barrierer for det at være fysisk aktiv under graviditeten, så stoppede de ikke. I stedet for skiftede de til nogle fysiske aktiviteter, som de magtede kropsligt, eller som de ikke var bekymrede for. De gravide kvinder fandt megen glæde og fysisk velbehag ved at være fysisk aktive under graviditeten.

KonklusionLigesom tidligere studier fandt delarbejde I en lavere risiko for en for tidlig fødsel blandt gravide kvinder, der var fysisk aktive i fritiden. Et nyt fund i denne afhandling var, at denne lavere risiko primært sås hos gravide kvinder med det mest intensive fysiske aktivitetsniveau i fritiden under graviditeten. Kvinder, der var fysisk aktive i fritiden før eller under graviditeten, fødte derimod børn med samme fødselsvægt og havde samme risiko for svangerskabsforgiftning som kvinder, der var fysisk inaktive. Gravide kvinder der dyrkede sport eller var fysisk aktive i fritiden havde samme risiko for at føde et barn med en høj fødselsvægt, som fysisk inaktive gravide kvinder. Sidstnævnte er ikke tidligere vist i et større studier. Adskillige studier har vist, at gravide kvinder oplever barrierer for at være fysisk aktive under graviditeten. Den nye dimension i det kvalitative studium var, at kvinderne oplevede, at disse barrierer kunne overvindes, således at de kunne fortsætte med at være fysisk aktive under graviditeten.På baggrund af denne afhandling fandt vi ingen tegn på, at gravides fysiske aktivitets niveau i fritiden var forbundet med negative helbreds udfald for fosteret eller moderen. Det kvalitative studie fandt derimod, at fysisk aktivitet under graviditeten var forbundet med såvel psykologiske som fysiske fordele for moderen, Endvidere tyder delarbejde I på, at gravide der er fysisk aktive i fritiden, har en lavere risiko for at føde for tidligt.

Denne afhandling giver således ikke grund til, at ændre på de danske anbefalinger om graviditet og fysisk aktivitet i fritiden.

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ACKNOWLEDGEMENTSThis thesis for the degree of Doctorate was carried out at the Department Health Sciences, Faculty of Medicine, Lund University. I would like to thank all those who have contributed to completion of this work and I wish to express my sincere gratitude to:

The more than 10,000 pregnant women who have participated in the “Aarhus Birth Cohort”, “Smoke-free Newborn Study” and “Danish Dystocia Study” and to the 19 women who shared their experiences and views with me in the interviews.

Professor Ingalill Rahm Hallberg for her helpfulness and willingness to accept me as doctoral student at the Department of Health Sciences.

Docent, Anna-Karin Dykes, main supervisor, for inspiring guidance and discussions and for introducing me to the exciting world of qualitative research.

Docent Kerstin Petersson, supervisor, for great guidance and brilliant pædagogical skills and for continuing as my supervisor.

Professor Peter Damm, supervisor, for his never–ending, ambitious, and thorough scientific guidance.

Head of Clinic Morten Hedegaard, supervisor, for his consistently great guidance and especially for discussions of epidemiological questions and statistical analyses.

Professor Bent Ottesen, supervisor, for encouraging me to undertake this thesis and his fruitful guidance and collaboration on funding applications and for, along with Center Chief Nurse Sanne Wilsdahl, giving me a great workspace at Juliane Marie Centre’s research unit.

Associate Professor Tine Brink Henriksen, Århus for allowing me to use data from the “Aarhus Birth Cohort” and her brilliant response to the manuscripts.

Professor Bente Klarlund Pedersen, for being such an excellent storyteller that I became inspired and dedicated to working with physical activity and pregnancy.

Midwife Hanne Kjærgaard for her encouragement and inspiration to begin this thesis at Lund University and thus follow in her footsteps.

Professor Bengt Saltin for fruitful discussions about his work about physical activity.

Rosemary Sørensen and Lisa Copple for editing papers and thesis. Secretary Pia Hynne for transcribing the qualitative interviews.

Administrative staff and IT teams in Lund and administrative staff and IT Teams Rigshospitalet especially Ann-Kristin Inglöff, Jonna Hemningsen,

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Susanne Mårtensson, Irene Skov, Horst Donnerstag and Peder Lave Pedersen for their kindness in helping with administrative problems.

All my colleagues in the research group Health and Care of Women and Children for constructive criticism of the manuscripts and to Professor Inger Hallström for leading the research group

All my colleagues at Juliane Marie Centre´s research unit valuable professional discussions.

To two other colleagues Line Rode and Mette Juhl. Close friends, my family and my dearest women friends. My heartfelt thanks to my two sons, Morten and Kristian, and my husband

Mikael for all their loving support and encouragement – it is a challenge to write a thesis in the midst of a family of four, all busy and ambitious.

When my aging father a year ago expressed his wish to live to see my dissertation, the process was accelerated. After his stroke last autumn, the work gained momentum and although he is not well enough to participate in Lund, he is able to read the entire thesis – and I am happy that his wish was fulfilled.

The study was supported by grants from the Research Committee of the Copenhagen Hospital Corporation, The Danish Association of Midwives, Aase and Ejnar Danielsen´s Foundation, Sophus Jacobsen and wife Astrid Jacobsen Foundation; The Augustinus Foundation, and Else and Mogens Wedell-Wedellsborg´s Foundation.

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REFERENCES

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Avhandlingar i ämnet vårdvetenskap med inriktning omvårdnad: kvinnor och barns hälsa från Institutionen för hälsa, vård och samhälle, Lunds universitet.

Wennick, Anne. Living with childhood diabetes. Family Experiences and Long-term effects. Department of Health Sciences, Faculty of Medicine, Lund University, Sweden 2007.

Kjaergaard, Hanne. Dystocia in nulliparous women. Incidence, outcomes, risk indicators and women’s experiences. Department of Health Sciences, Faculty of Medicine, Lund University, Sweden 2007.

Björk, Maria. Living with childhood cancer. Family Members´ Experiences and Needs. Department of Health Sciences, Faculty of Medicine, Lund University, Sweden 2008.

Ekelin, Maria. Parents´expectations, experiences and reactions to routine ultrasound examination during pregnancy. Department of Health Sciences, Faculty of Medicine, Lund University, Sweden 2008.

Lundqvist, Pia. Children Born Prematurely. Their fathers´ experiences and trends in mortality and morbidity during a ten-year period. Department of Health Sciences, Faculty of Medicine, Lund University, Sweden 2008.

Larsson, Anna-Karin. Parent’s experiences and reactions when an unexpected finding in their foetus is revealed at a routine ultrasound examination – a multi method study. Department of Health Sciences, Faculty of Medicine, Lund University, Sweden 2009.

Hegaard, Hanne Kristine. Pregnancy and leisure time physical activity. Department of Health Sciences, Faculty of Medicine, Lund University, Sweden 2009.

Ovanstående kan rekvireras från

Lunds universitetInstitutionen för hälsa, vård och samhälleBox 157, 221 00 Lund

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