prior cs and oasi incidence

ORIGINAL ARTICLE

A prior cesarean section and incidence of obstetric analsphincter injury

Sari Räisänen & Katri Vehviläinen-Julkunen &

Rufus Cartwright & Mika Gissler & Seppo Heinonen

Received: 12 September 2012 /Accepted: 12 November 2012# The International Urogynecological Association 2012

AbstractIntroduction and hypothesis Obstetric anal sphincter injury(OASIS) following birth may have serious, long-termeffects on affected women, including fecal incontinence,despite primary repair.Methods This was a retrospective population-based registerstudy. Women with OASIS grouped by order of vaginaldelivery and prior cesarean section (CS) were compared sep-arately with women without OASIS using logistic regressionanalysis. The aim was to assess an association between priorCS and incidence of OASIS across groups of women catego-rized according to singleton first, second, and third vaginaldeliveries between 1997 and 2007 in Finland.Results The incidence of OASIS was 1.8 % at a first vaginaldelivery after a prior CS compared with 1.0 % at a firstvaginal delivery without prior CS. After adjustment priorCS was associated with a 1.42-fold risk of OASIS only atthe first vaginal delivery, with no further significant riskafter one or two previous vaginal deliveries. One centimeterincrease in maternal height was associated with a 2 % de-crease in OASIS incidence at the first vaginal delivery.

Conclusions Prior CS is a significant risk factor for OASISat the first vaginal delivery. This suggests that relativefetopelvic disproportion leading to CS for a first deliveryalso predisposes to OASIS at a first vaginal delivery since40 % of the increased incidence of OASIS risk wasexplained by birthweight and 4 % by maternal height.

Keywords Birth injuries . Delivery, obstetric . Episiotomy .

Perineum, injuries . Registries . Cesarean section

Introduction

Obstetric anal sphincter injury (OASIS) is a serious com-plication of vaginal delivery that may result in analincontinence in one third to two thirds of women [1].There is a wide variation in the reported incidence ofOASIS between countries, ranging from 0.2 to 3.5 % in2004 in Europe [2] and from 3.5 to 5.9 % in the USA[3, 4]. An overall increased incidence has been reportedin recent decades, which may be partially explained by

S. Räisänen (*)Savonia University of Applied Sciences, P.O. Box 72,74101 Iisalmi, Finlande-mail: [email protected]

K. Vehviläinen-JulkunenDepartment of Nursing Science, University of Eastern Finland,P.O. Box 1627, 70211 Kuopio, Finlande-mail: [email protected]

K. Vehviläinen-JulkunenKuopio University Hospital, P.O. Box 1777,70211 Kuopio, Finland

R. CartwrightInstitute of Reproductive and Developmental Biology,Hammersmith Hospital, Du Cane Road,London W12 0NN, UKe-mail: [email protected]

M. GisslerNational Institute for Health and Welfare (THL),P.O. Box 30, Lintulahdenkuja 4,00271 Helsinki, Finlande-mail: [email protected]

M. GisslerNordic School of Public Health, Gothenburg, Sweden

S. HeinonenDepartment of Obstetrics and Gynaecology, Kuopio UniversityHospital, P.O. Box 1777, 70211 Kuopio, Finlande-mail: [email protected]

S. HeinonenUniversity of Eastern Finland, P.O. Box 1627, 70211 Kuopio,Finland

Int Urogynecol JDOI 10.1007/s00192-012-2006-6

improvements in either diagnosis or routine registration[5, 6].

Consistently identified risk factors for OASIS based onprevious studies include first vaginal birth [7, 8], high birth-weight [9, 10], prolonged active second stage of birth [8],assisted deliveries [7, 10], and midline episiotomy [3, 11,12]. Data regarding mediolateral and lateral episiotomy areless consistent, but large population-based studies demon-strate decreased risk of OASIS [8, 10, 13]. Lateral episiot-omy, which is performed in the vaginal introitus 1 or 2 cmlateral to the midline and directed downwards towards theischial tuberosity, is exclusively used in Finland [14]. Someprevious studies have considered an association betweenprevious cesarean section (CS) and incidence of OASIS. Afirst vaginal delivery after a prior CS has been reported to beassociated either with a similar [15] or an increased inci-dence of OASIS [3, 7, 16] compared to a first vaginaldelivery without a prior CS. In addition, two studies fromthe USA, where midline episiotomy is preferred, reported anincreased association between the incidence of OASIS andfirst vaginal deliveries after a prior CS compared to firstvaginal deliveries [3, 16]. An excess risk from prior CS hasalso been reported for second and subsequent vaginal deliv-eries [3], including for the subset of women deliveringspontaneously [7].

It has been clearly established that the risk of OASISdeclines with increasing parity, with some threshold effect,evident at the first vaginal delivery [8–10]. Conclusionsfrom previous studies are limited, both by the absence ofcomparisons right across the range of vaginal birth order andrisk of unmeasured confounding factors associated with CSat the first birth. Accordingly, the aim of the present studywas to assess an association between prior CS and incidenceof OASIS across groups of women categorized according tosingleton first, second, and third vaginal deliveries between1997 and 2007 in Finland.

Materials and methods

The data were gathered from the Finnish Medical BirthRegister with linkage to the Hospital Discharge Registerthat are both maintained by the National Institute for Healthand Welfare (THL) in Finland. Permission to use the confi-dential register data in this study was granted 16 October2008 by the THL in Finland (reference number 2777/605/2007). The Medical Birth Register established in 1987includes information on maternal and neonatal birth charac-teristics and perinatal outcomes concerning all live births orstillbirths delivered after the 22nd gestational week orweighing 500 g or more.

Information on OASIS was not collected in the MedicalBirth Register prior to 2004, so for the years 1997–2003, the

information about OASIS was taken from the HospitalDischarge Register, based on the International Classificationof Diseases (ICD-10) codes O70.2 (third degree) and O70.3(fourth degree). The Hospital Discharge Register establishedin 1969 contains information on all aspects of inpatient careand outpatient visits in Finnish hospitals; thus, we had alsoinformation concerning all aspects of care during pregnancyand birth such as medical interventions and surgical proce-dures. The two data sources were linked together usingencrypted unique personal identification numbers. Womenwith only a first OASIS were included and those withsubsequent OASIS (n022) were excluded from analysis.The use of a sensitive electronic health register for theperiod from 1997 to 2007 required authorization by nationaldata protection legislation. Only anonymized data were usedand consequently the informed consent of the registeredindividuals was not needed.

The data included all women with singleton first to thirdvaginal deliveries (n0463,918) of which 6.2 % (n028,942)had a prior CS. The deliveries were categorized into sixgroups, which were first vaginal, first vaginal delivery aftera prior CS, second vaginal delivery, second vaginal deliveryafter a prior CS, third vaginal delivery, and third vaginaldelivery after a prior CS in univariate analyses.

Statistical differences between the subjects and the refer-ence group in the maternal and neonatal birth characteristicsand perinatal outcomes were evaluated by the chi-squaretest. The differences between groups in the continuous var-iables were evaluated by Student’s t and by Mann–WhitneyU tests as appropriate. In multivariate analyses deliverieswere grouped and analyzed by order of vaginal delivery inthree groups (first, second, and third vaginal delivery). Foreach vaginal delivery group logistic regression analyseswere used to calculate the odds ratio (OR) of OASIS byadjusting for both clinically relevant and statistically signif-icant factors (p<0.1) associated with OASIS. Furthermore,in order to examine whether mode of delivery and back-ground characteristics (maternal age, birthweight, maternalheight and weight) contributed to the risk of OASIS at thefirst vaginal delivery with and without a prior CS, weestimated the contribution of each of these factors by usinglogistic regression for the years 2004–2007. Maternalheight, maternal weight, length of active second stage ofbirth, and head circumference were available for inclusion inlogistic regression analyses for births that occurred after2003. Each intervention or background characteristics wereadded separately to model B (risk adjusted by a prior CS andmaternal age) and the contribution of each factor was mea-sured by the percentage reduction in the OR of OASIScompared to the model B (OR model B−OR model C/D/E/F)/(OR model B−1) [17, 18].

The degree of OASIS was classified according to stan-dard definitions: a third-degree rupture involves the external

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anal sphincter and a fourth-degree rupture affects both theanal sphincter and the anorectal mucosa [19]. In all of theanalyses, data on third- and fourth-degree OASIS werepooled. Active second stage of birth was defined as com-mencement of active pushing until delivery of the infant.Apart from birthweight, length of active second stage ofdelivery, head circumference, maternal height, and pregra-vid body mass index (BMI), all other variables were cate-gorical or dichotomous in multivariate analyses. BMI wascalculated by dividing body weight in kilograms by heightin meters squared. Differences were deemed to be signifi-cant if p<0.05. The data were analyzed using SPSS forWindows 19.0 (Chicago, IL, USA).

Results

For women with a singleton vaginal delivery in 1997–2007,the overall incidence of OASIS was 0.6 % (n02,786 of463,086). As expected rates of OASIS declined acrossgroups of vaginal delivery from first to third. The incidenceof OASIS at a first vaginal delivery (women without a priorCS) was 1.0 % compared with 1.8 % at a first vaginaldelivery after a prior CS (p≤0.001). There was no measur-able absolute difference in subsequent deliveries, with anOASIS incidence of 0.2 % at second vaginal deliveries(without and with a prior CS, p00.33) and 0.1 % at thirdvaginal deliveries (without and with a prior CS, p00.22)(Tables 1, 2, and 3). Unadjusted OR and 95 % confidenceintervals (CI), with first vaginal delivery (including womenwith prior CS) as the reference group, were OR 0.21 (95 %CI 0.19–0.23) and OR 0.08 (95 % CI 0.06–0.10), respec-tively (data not shown).

After adjustment for confounders and mediators a priorCS was associated with a 1.42-fold incidence of OASIS atfirst vaginal deliveries (adjusted OR 1.42, 95 % CI 1.25–1.61), with no significant association at the second and thethird vaginal deliveries (adjusted ORs 0.91, 95 % CI 0.59–1.43 and 1.32, 0.48–3.63, respectively) as shown in Table 4.Instrumental assisted delivery was associated with an in-creased incidence of OASIS at first and second vaginaldeliveries and high birthweight across all groups of vaginaldeliveries.

The use of lateral episiotomy was associated with a 25 %(adjusted OR 0.75, 95 % CI 0.68–0.82) decreased incidenceof OASIS at first vaginal deliveries (without and with aprevious CS), whereas use of the procedure was associatedwith a significantly increased incidence of OASIS at thirdvaginal deliveries. The length of active second stage of birthwas associated with an increased incidence of OASIS at thefirst and second vaginal deliveries, with a similar but non-significant trend at third vaginal deliveries, probably due tothe low number of cases. At first vaginal deliveries,

maternal height was significantly negatively associated withthe incidence of OASIS. One centimeter increase in mater-nal height was associated with a 2 % decrease in the inci-dence of OASIS (adjusted OR 0.98, 95 % CI 0.97–0.99),such that a 10-cm increase in maternal height was associatedwith a 20 % relative decreased incidence of OASIS. Further,it appeared that the mean height (±SD) of women at firstvaginal deliveries with a prior CS was lower than thosewithout it [165.2 cm (±5.9) vs 166.0 cm (±5.9), p≤0.001],as shown in Table 5.

Furthermore, we measured the contribution of mode ofdelivery and background characteristics to the incidence ofOASIS associated with a prior CS before a first vaginaldelivery using a percentage reduction in the OR, as shownin Table 6. For example, the OR of OASIS adjusted by aprior CS and maternal age was 1.53 (95 % CI 1.35–1.73)and after birthweight was added to the model the OR de-creased to 1.32 (95 % CI 1.17–1.49). This means that39.7 % of the OASIS incidence at first vaginal delivery aftera prior CS can be explained by birthweight and 3.8 % bymaternal height.

Discussion

The present study aimed to assess an association betweenprior CS and incidence of OASIS across groups of womencategorized according to singleton first, second, and thirdvaginal deliveries between 1997 and 2007 in Finland. Themain finding was that a prior CS was associated with a 1.42-fold increased risk of OASIS at the first vaginal delivery,with no significant association in subsequent vaginal deliv-eries. The incidence of OASIS at first vaginal deliveries was1.1 % (2,354 of 221,347) and declined substantially atsecond (0.2 %, 368 of 165,960) and third deliveries(0.1 %, 66 of 76,611). This inverse association betweenincidence of OASIS and vaginal birth order confirmed theresults of previous large retrospective population-basedstudies from the Netherlands and Norway. In both thesecountries mediolateral episiotomy is preferred, and the over-all incidence of OASIS is approximately twofold higherthan in Finland [7, 10].

The most important strength of this analysis was that thedata were gathered from two high-quality mandatory nation-al registers, each of which provides excellent coverage ofthe entire Finnish population with validated data quality [5,20]. Nevertheless, it might be a concern that this kind ofregister includes errors and missing values, because the dataare produced mainly for administrative and statistical pur-poses, not primarily for research. For the period 2006–2007,we were able to compare our two data sources, when therewas independent recording of OASIS in both registers.During this period the Hospital Discharge Register covered

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95 % of OASIS cases recorded in the Medical Birth Regis-ter, which adds to our confidence in the reliability of caseascertainment. For the period 2004–2007 we were able toadjust for a wide range of potential confounders and medi-ators, including maternal height, BMI, length of active sec-ond stage of birth, and head circumference, which by meansof multivariate modeling strengthens the association be-tween a prior CS and OASIS.

Among the possible limitations of this analysis were thelack of data regarding the number of previous CS for wom-en with three or more deliveries. Although a vaginal deliv-ery after two CS is rarely and after three CS never

recommended in Finland, there might have been a few suchcases, in which we would have underestimated vaginalparity. This would tend to bias away from the null resultswe report for an effect of prior CS on second and thirdvaginal deliveries. In the present study, there were 528 caseswith missing information on parity, which were excludedfrom the data. This exclusion, however, is unlikely to haveaffected our results, since their share was only 0.1 % of thetotal population.

Our results pertaining to an association between a priorCS and an incidence of OASIS confirmed the results ofprevious studies and were of similar magnitude [3, 7, 16].

Table 1 Delivery characteristics and interventions for singleton first vaginal delivery and first vaginal delivery after a prior CS for the first birth(chi-square, Mann-Whitney U, marked with *, and Student’s t tests, marked with **)

Delivery intervention/characteristic,% or mean (±SD)

First vaginal delivery, n0204,104 p value First vaginal delivery after a priorCS for their first birth, n017,243

p value

With OASIS(1.0 %, n02,046)

Without OASIS With OASIS(1.8 %, n0308)

Without OASIS

Mean maternal age, years (±SD) 28.1 (±4.8) 26.8 (±5.1) ≤0.001 31.1 (±4.1) 30.3 (±4.7) 0.001**

≤19 3.4 6.9 ≤0.001 0.0 0.5 0.00820–29 59.8 64.0 34.7 42.8

30–39 35.6 28.0 63.6 54.3

≥40 1.2 1.1 1.6 2.4

Mean maternal height, cm (±SD)a 165.8 (±5.9) 166.0 (±5.9) 0.78 165.1 (±5.6) 165.2 (±5.9) 0.95**

BMI (±SD)a 23.6 (±4.2) 23.4 (±4.3) 0.14 24.7 (±4.7) 24.5 (±4.7) 0.60**

Mode of delivery

Vaginal spontaneous 63.1 84.2 ≤0.001 72.1 83.6 ≤0.001Breech 0.5 0.8 0.3 0.3

Forceps 0.8 0.2 1.3 0.2

Vacuum assistance 35.5 14.8 26.3 15.9

Mean length of active 2nd stage ofbirth, min (±SD)a

63.9 (±57.2) 42.5 (±42.5) ≤0.001 54.1 (±49.3) 38.7 (±40.7) ≤0.001**

≤15 11.6 23.7 15.4 27.8 ≤0.00116–30 22.5 31.0 26.5 32.4

31–45 20.5 18.9 23.5 16.9

46–60 15.2 10.5 9.6 9.4

≥61 30.1 15.9 25.0 13.5

Mean birthweight, g (±SD) 3,677.8 (±445.6) 3,417.0 (±445.7) ≤0.001 3,795.9 (±448.8) 3,574.2 (±514.5) ≤0.001*

<3,000 6.2 15.7 ≤0.001 3.9 10.5 ≤0.0013,000–3,499 28.8 37.4 20.1 30.7

3,500–3,999 42.1 34.7 40.9 39.7

≥4,000 22.9 12.2 35.1 19.0

Mean head circumference, cm (±SD)a 35.2 (±1.4) 34.7 (±1.6) ≤0.001 35.4 (±1.4) 35.1 (±1.6) 0.03**

Induction 17.0 15.2 0.03 20.1 17.9 0.30

Augmentation with oxytocin 65.8 63.0 0.009 65.9 61.3 0.10

Episiotomy 65.7 63.5 0.04 55.8 63.2 0.008

Epidural analgesia 58.9 60.7 0.10 63.0 61.0 0.48

Nitrous oxide gas 53.2 56.8 0.001 51.0 55.2 0.14

Paracervical block 11.7 14.6 ≤0.001 12.3 15.0 0.11

aMaternal height, BMI, length of active 2nd stage of birth, and head circumference covered only for the years 2004–2007. Information for the years2004–2007: n083,179 of which 6,568 had a prior CS, n OASIS01,379 of which 174 had a prior CS

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However, the same association in the second and thirdvaginal deliveries was not in line with previous studies thathave suggested that a prior CS increased the risk of OASISregardless of the number of previous vaginal deliveries [3,7]. Our results demonstrated that in the Finnish population aprior CS was not associated with increased incidence ofOASIS at the second and third vaginal deliveries. Thesefindings could be explained by the fact that in the previousstudies the risk of OASIS was adjusted among the totalpopulation, whereas in our study we were able to adjustamong strata of women with equal number of vaginal de-liveries. Other differences may arise from wide variation in

obstetric practice with regard to CS. In 2004 CS rates inEurope varied from 14.3 % in Slovenia to 37.8 % in Italy[2], whereas in 2009 the CS rate in the USA was 32.9 %[21]. In Finland, the annual CS rate has been quite constantand varied from 15.2 to 16.2 % during the study period.Differences in risk associated with prior CS may thereforereflect a different risk profile in different settings.

Based on results of multivariate analyses we suggest thatincreased incidence of OASIS after a prior CS at firstvaginal delivery might reflect relative fetopelvic dispropor-tion. Such fetopelvic disproportion might predispose to CSat a first delivery and subsequently contribute to increased

Table 2 Delivery characteristics and interventions for singleton second vaginal delivery and second vaginal delivery after a prior CS for the firstbirth (chi-square and Mann-Whitney U)

Delivery intervention/characteristic Second vaginal deliveries, n0157,317 p value Second vaginal delivery + prior CS, n08,296 p value

With OASIS (0.2 %,n0347), % or mean

Without OASIS,% or mean



Mean maternal age, years (±SD) 30.9 (±4.6) 29.4 (±4.8) ≤0.001 33.2 (±3.7) 32.0 (±4.5) 0.22*

≤19 0.0 0.8 ≤0.001 0.0 0.1 0.1920–29 38.9 50.9 9.5 29.1

30–39 55.9 46.4 81.0 66.5

≥40 5.2 1.9 9.5 4.3

Mean maternal height, cm (±SD)a 165.3 (±5.8) 166.1 (±5.9) 0.11 164.9 (±4.1) 165.1 (±5.7) 0.96

BMI (±SD)a 23.9 (±3.9) 24.1 (±4.5) 0.58 23.2 (±2.3) 24.9 (±4.7) 0.39

Mode of delivery

Vaginal spontaneous 87.6 97.1 ≤0.001 90.5 95.2 0.66Breech 0.6 0.6 0.0 0.5

Forceps 0.3 0.0 0.0 0.1



29.4 (±34.2) 14.8 (±19.7) ≤0.001 15.6 (±16.5) 16.1 (±21.9) 0.80

≤15 39.0 72.0 ≤0.001 62.5 69.2 0.3016–30 35.6 19.1 25.0 20.4

31–45 13.7 5.1 0.0 5.9

46–60 1.4 1.9 12.5 2.1

≥61 10.3 2.0 0.0 2.4

Mean birthweight, g (±SD) 3,953.4 (±472.3) 3,617.1 (±499.0) ≤0.001 3,932.4 (±441.7) 3,616.7 (±560.5) 0.003

<3,000 2.3 9.0 ≤0.001 4.8 10.5 0.023,000–3,499 12.4 29.9 4.8 27.3

3,500–3,999 40.9 40.1 42.9 38.7

≥4,000 44.4 21.1 47.6 23.4

Mean head circumference, cm (±SD)a 35.5 (±1.4) 35.01 (±1.4) ≤0.001 35.7 (±1.8) 35.0 (±1.8) 0.37

Induction 17.0 13.6 0.07 28.6 21.1 0.40

Augmentation with oxytocin 41.8 33.3 ≤0.001 42.9 39.2 0.73

Episiotomy 30.5 20.8 ≤0.001 33.3 24.6 0.35



Paracervical block 23.3 24.1 0.75 19.0 23.4 0.64

aMaternal height, BMI, length of active 2nd stage of birth, and head circumference covered only for the years 2004–2007. Information for the years2004–2007: n060,861 of which 3,088 had a prior CS, n OASIS0208 of which 13 had a prior CS

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risk of OASIS. Because we had no direct information onpelvic parameters or clinical suspicion of cephalopelvicdisproportion, we used maternal height as a surrogatevariable. Several studies have established maternal heightto be correlated to the size of the pelvis, and womenwith cephalopelvic disproportion are shorter compared tothose with spontaneous vaginal deliveries.[22–26] Indeed,in this sample, the mean maternal height of women withfirst vaginal delivery after a prior CS was 0.8 cm lowerthan those without a prior CS. Furthermore, in multivar-iate analysis a 1 cm increase in maternal height was

associated with a 2 % decrease in the incidence ofOASIS. Thus, for example, a 5-cm increase in maternalheight was associated with a 10 % decrease on OASISrates. Although we were also able to adjust both forbirthweight and head circumference, cephalopelvic dis-proportion can never be predicted with certainty basedon either maternal or fetal measures, because it resultsfrom diverse factors including malposition of the fetalhead as well as contraction frequency and strengths[27]. So although we observed that women with a priorCS had an increased unadjusted risk of OASIS, based on

Table 3 Delivery characteristics and interventions for singleton third vaginal delivery and third vaginal delivery after a prior CS for the first birth(chi-square and Mann-Whitney U)

Delivery intervention/characteristic

Third vaginal deliveries, n073,347 p value Third vaginal delivery + prior CS,n03,264

p value





Mean maternal age, years (±SD) 33.7 (±4.6) 31.6 (±4.7) ≤0.001 38.0 (±3.7) 33.2 (±6.6) 0.03

≤19 0.0 0.1 0.008 0.0 0.0 0.2920–29 14.5 32.7 75.0 24.8

30–39 75.8 62.7 25.0 67.3

≥40 9.7 4.5 0.0 8.0

Mean maternal height, cm (±SD)a 165.3 (±7.0) 165.9 (±5.8) 0.70 174.0 164.9 (±5.7) 0.13

BMI (±SD)a 25.3 (±4.6) 24.4 (±4.7) 0.15 24.8 25.3 (±4.9) 0.87

Mode of delivery

Vaginal spontaneous 95.2 97.8 0.19 75.0 97.0 0.04Breech 0.0 0.6 0.0 0.5

Forceps 0.0 0.0 0.0 0.0



25.4 (±25.4) 11.2 (±15.8) ≤0.001 42.0 12.0 (±16.9) 0.10

≤15 50.0 81.3 ≤0.001 0.0 80.0 ≤0.00116–30 16.7 13.3 0.0 14.8

31–45 12.5 3.1 100.0 2.9

46–60 16.7 1.1 0.0 0.7

≥61 4.2 1.1 0.0 1.6

Mean birthweight, g (±SD) 3,934.1 (±519.9) 3,660.3 (±515.7) ≤0.001 3,861.3 (±375.6) 3,621.2 (±580.8) 0.36

<3,000 1.6 8.2 0.001 0.0 11.0 0.443,000–3,499 14.5 27.2 0.0 26.2

3,500–3,999 38.7 39.8 50.0 38.4

≥4,000 45.2 24.7 50.0 24.4

Mean head circumference, cm (±SD)a 35.6 (±1.3) 35.1 (±1.4) 0.05 – 35.1 (±1.6)

Induction 24.2 18.0 0.21 25.0 25.1 1.00

Augmentation with oxytocin 40.3 31.7 0.15 25.0 39.0 0.57

Episiotomy 22.6 9.3 ≤0.001 50.0 10.0 0.008



Paracervical block 32.3 24.7 0.17 0.0 25.6 0.24

aMaternal height, BMI, length of active 2nd stage of birth, and head circumference covered only for the years 2004–2007. Information for the years2004–2007: n026,922 of which 1183 had a prior CS, n OASIS033 of which 2 had a prior CS

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both measurable maternal and fetal factors (Table 5), weconclude that there was also an excess risk resulting fromunmeasured but persistent above-mentioned common contrib-utors both to CS and OASIS.

The results of the present study are applicable tocountries with a similar incidence of OASIS but may bevery different in countries with markedly lower or higherOASIS or CS rates, or other differences in obstetric practice.

In particular, low use of forceps and exclusive use of lateralepisiotomy may reduce generalizability.

Conclusions

A prior CS was associated with a 1.42-fold incidence ofOASIS at first vaginal deliveries, even after adjustment for a

Table 4 Adjusted OR of OASIS at first, second, or third singleton vaginal delivery between 1997 and 2007 in Finland (logistic regressionanalyses)

Delivery intervention/characteristic First vaginal delivery Second vaginal delivery Third vaginal deliveryOR (95 % CI), n0221,347(2004–2007, n040,022)

OR (95 % CI), n0165,892(2004–2007, n029,366)

OR (95 % CI), n076,580(2004–2007, n013,499)

Maternal age

≤19 1

20–29 (ref. ≤29 in multiparous) 1.71 (1.34–2.17)*** 1 1

30–39 2.18 (1.70–2.78)*** 1.53 (1.23–1.91)*** 2.56 (1.25–5.21)**

≥40 1.68 (1.08–2.60)* 3.43 (2.13–5.53)*** 4.70 (1.73–12.76)**

Maternal height, cma 0.98 (0.97–0.99)** 0.98 (0.95–1.02) 0.95 (0.87–1.03)

BMIa 0.99 (0.97–1.01) 0.96 (0.76–1.10) 0.97 (0.87–1.09)

Mode of delivery

Vaginal spontaneous 1 1 1

Breech 1.16 (0.64–2.11) 1.66 (0.41–6.73) –

Forceps 4.90 (3.15–7.64)*** 8.97 (1.21–66.16)*** –

Vacuum assistance 2.90 (2.62–3.16)*** 3.81 (2.70–5.39)*** 2.18 (0.76–6.23)

Length of active 2nd stage of birth per 1 h increasea 1.22 (1.12–1.33)*** 1.44 (1.12–1.84)*** 1.60 (0.84–3.06)

Birthweight per 1,000 g increase 2.62 (2.40–2.86)*** 3.95 (3.19–4.90)*** 2.69 (1.64–4.43)***

Head circumference, cma 1.03 (0.96–1.10) 0.91 (0.75–1.12) 0.84 (0.54–1.32)

Induction 1.02 (0.91–1.13) 0.93 (0.72–1.21) 1.23 (0.70–2.17)

Augmentation with oxytocin 1.07 (0.97–1.17) 1.06 (0.84–1.33) 1.01 (0.66–1.83)

Episiotomy 0.75 (0.68–0.82)*** 1.12 (0.89–1.42) 2.31 (1.29–4.16)**

Epidural analgesia 0.78 (0.71–0.86)*** 1.02 (0.80–1.30) –

Nitrous oxide gas 0.86 (0.74–0.94)** – –

Paracervical block 0.67 (0.59–0.77)*** – –

Prior CS 1.42 (1.25–1.61)*** 0.91 (0.59–1.43) 1.32 (0.48–3.63)

*p value<0.05; **p<0.01; ***p<0.001aMaternal height, BMI, length of active 2nd stage of birth, and head circumference adjusted only for the years 2004–2007

Table 5 Characteristics ofwomen with or without prior CSat first singleton vaginal deliveryduring 1997–2007 in Finland(chi-square, Mann-Whitney U,or Student’s t tests)

aMaternal height, maternalweight, and head circumferencewere analyzed only for the years2004–2007

Characteristic First vaginal delivery, n0221,347 (in 2004–2007, n049,327–74,220)

p value

With a prior CS Without a prior CS

Mean maternal height, cm (±SD)a 165.2 (±5.9) 166.0 (±5.9) ≤0.001

Mean maternal weight, kg (±SD)a 67.1 (±13.6) 64.7 (±12.7) ≤0.001

Mean birthweight, g (±SD) 3577.7 (±515.1) 3448.0 (±502.7) ≤0.001

Mean head circumference, cm (±SD)a 35.1 (±1.6) 34.7 (±1.6) ≤0.001

Int Urogynecol J

wide range of risk factors for both CS and OASIS. At secondand third vaginal deliveries there was no significant associa-tion. Maternal height, which correlates with pelvic diameters,and birthweight were strongly associated with incidence ofOASIS and in the order of 44 % of increased incidence ofOASIS at first vaginal deliveries after a prior CS. Thus, a priorCS might reflect relative fetopelvic disproportion that persistsin subsequent deliveries and increases the risk of OASIS.

Conflicts of interest None.

References

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able

6OR

forOASIS

atfirstvaginaldeliv

erieswith

andwith

outapriorCSafteradjustmentby

interventio

nsandcharacteristics(n082

,031

)between20

04and20

07in

Finland

(log

istic

regression

)

Mod

elA

Mod

elB

Mod

elC

Mod

elD

Mod

elE

Mod

elF

OR(95%

CI)

OR(95%

CI)

OR(95%

CI)

Diff.with

B(%

)aOR(95%

CI)

Diff.with

B(%

)aOR(95%

CI)

Diff.with

B(%

)aOR(95%

CI)

Diff.with

B(%

)a

ApriorCS

1.78

(1.58–2.01

)1.53

(1.35–1.73

)1.57

(1.39–1.77

)–

1.32

(1.17–1.49

)39

.71.51

(1.28–

1.80

)3.8

1.52

(1.28–1.80

)1.9

Mod

elA0adjusted

byapriorCS;mod

elB0adjusted

byapriorCSandmaternalage;mod

elC0adjusted

byapriorCS,m

aternalage,andmod

eof

deliv

ery;

mod

elD0adjusted

byapriorCS,

maternalage,andbirthw

eigh

t;mod

elE0adjusted

byapriorCS,maternalage,andmaternalheight;mod

elF0adjusted

byapriorCS,maternalage,andmaternalweigh

taThe

risk

ofOASIS

atfirstv

aginaldeliv

erieswith

andwith

outa

priorCSwas

adjusted

bymaternalage

only

inmod

elB.E

achinterventio

nor

demog

raph

iccharacteristicswereaddedseparately

tomod

elB,andthecontribu

tionof

each

factor

was

measuredby

thepercentage

redu

ctionin

theORof

anal

rupturecomparedto

mod

elB(O

Rmod

elB−O

Rmod

elC/D/E/F)/(O

Rmod

elB−1

)

Int Urogynecol J

http://www.europeristat.com/

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prior cs and oasi incidence

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