first birth timing in australia

FIRST BIRTH TIMING IN AUSTRALIAAuthor(s): Rosangela MerloSource: Journal of the Australian Population Association, Vol. 12, No. 2 (November 1995), pp.131-146Published by: SpringerStable URL: http://www.jstor.org/stable/41110402 .

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Vol. 12, No.2, 1995 Journal of the Australian Population Association

FIRST BIRTH TIMING IN AUSTRALIA*

Rosangela Merlo Graduate Studies in Demography

National Centre for Development Studies The Australian National University

Canberra ACT 0200

This paper examines the concept of delayed childbearing in Australia, in comparison with other Western countries. In addition to presenting statistics to examine changes in the age at which women enter parenthood, survey data from the Australian Family Project are used to investigate the factors influencing the timing of the first birth. Using a framework proposed by Bloom (1984), the paper presents a proportional hazards regression model of first birth timing. Some attempt is made to examine changes over time in the factors affecting the age at first birth.

There has been considerable interest among demographers, economists and sociologists in the transition to parenthood and changes in the timing of the first birth. The significance of this area of analysis is evident on a number of levels. The arrival of the first child marks a major turning point in the life course of parents (Miller and Newman 1978:2), and advances in effective contraception have facilitated a far greater degree of control over the transition to parenthood than ever before. Thus it has been argued that the timing of the first birth is an important social indicator since it signals a woman's prepared- ness to accept the role of mother (Bloom 1984). First birth timing also has important social and economic implications for both the prospective mother and her family. Transition to motherhood affects a woman's opportunities to further her education and career prospects (Bloom 1984; Teachman and Polonko 1985), and it may restrict her participation in other activities such as travel and recreation (Miller and Newman 1978). Economically, it has been shown that early first birth timing can lead to couples having less savings and property, and lower incomes (Coombs and Freedman 1966, cited in Wilkie 1981:583).

At the macro level, the timing of the first birth has important implications for population growth, as delayed childbearing can affect subsequent fertility by reducing the reproductive life span. Past research has demonstrated an inverse relationship between the timing of the first birth and completed family size. Women postponing parenthood will generally have fewer children than those commencing childbearing earlier (Bumpass et al 1978, cited in Wilkie 1981:583; Rindfiiss et al 1980 cited in Bloom 1982a:367; Choi and Ruzicka 1987: 133). Postponed births also affect population growth by lengthening the

Revised version of an essay awarded the W.D. Borne prize (undergraduate section), 1994.

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time until the next generation reaches childbearing age (Coale and Tye 1961, cited in Wilkie 1981:587).

This paper examines evidence of changing trends in the timing of first births among Australian women. Results of studies of childbearing delay in other Western countries are presented for comparative purposes. After discussing some methodological issues involved in measuring delayed childbearing, the paper presents findings from vital statistics and a recent national survey. Possible explanatory factors for delayed childbearing are identified and their significance assessed using a proportional hazards regression model, based on a conceptual scheme proposed by Bloom (1984). Finally, some attempt is made to identify whether these explanatory factors have changed over time.

Evidence of Postponed Childbearing There is considerable evidence, both from Australia and from other

Western countries, that a change in the timing of first birth has occurred. Wilkie (1981:583), for example, has found that nuptial first births in the United States during the years 1960-64 occurred, on average, 14 months after marriage, and the interval increased to 24 months during 1975-78. The trend toward later childbearing is also evident in Germany, particularly among women born after 1950 (Tuma and Huinink 1990), and the mean age at first birth of female cohorts in Austria, Finland and Portugal has also been increasing (Bloom 1982b). Cumulative first order birth rates reveal that Canadian women born in 1951 experienced a major change in the timing of first births, and each subsequent cohort displayed a lower proportion of women entering motherhood before the age of 25 (Ram 1990:151).

In Australia, Ruzicka found that, excluding brides pregnant at marriage, the proportion of brides postponing childbearing beyond two years of marriage increased from around 58 per cent of women who married in 1940- 44 to about 67 per cent for those married in 1965-69, and was higher still among the 1970-71 marriage cohort, especially among brides aged between 18 and 24 at marriage (Ruzicka 1976:535-536). Choi and Ruzicka found that the median age of Australian mothers at first nuptial births increased from 23.9 years in 1971-75 to 26.6 years in 1985. In addition, their examination of the interval between marriage and the first birth showed that the proportion of married women who have their first child within three years of marriage (excluding pregnant brides) has declined steadily since 1956 - the most obvious decline occurring between 1966 and 1970. They indicate that age specific fertility rates have declined most notably among women aged 15-19 and 20-24 years, while these rates have increased among 30-34 and 35-39 year old women.

This finding is perhaps best illustrated by Figure 1 which summarizes the distribution of first nuptial births by age of mother in each calendar year from 1908 to 1992. The proportion of women having their first nuptial birth before their 25th birthday remained relatively constant (at around 50 per cent) throughout the first half of this century. Minor fluctuations can be attributed in part to period factors such as the two World Wars and the Great

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Depression of the 1930s. The increase in first-time mothers aged under 25 during the baby boom period from 1946 to the mid-1960s is followed by a striking decline in the proportion of first births to women under 25 years. In 1992, only 24 per cent of first births were attributable to women under 25, after being as high as 69 per cent in 1968. The proportion of first births to women at all older age groups increased - the most notable change occurring among women aged 30-34 years.

Figure 1 Distribution of first nuptial births by age of mother, Australia, 1908- 1992

Sources: Australian Bureau of Statistics Births Bulletins; Commonwealth Bureau of Census and Statistics Population and Vital Statistics, Australian Demography and Demography Bulletins.

Methodological Considerations It should be noted that the vital statistics data presented above are limited to

nuptial first births. This has a twofold adverse effect on the quality of data. Firstly, official statistics in Australia cross-classify births by the mother's previous issue within the current marriage only. Therefore, women with children from a previous partnership who remarry and subsequently have children will be classified as having two 'first births'. This may artificially inflate the overall trend to delayed childbearing. In 1991, for example, some nine per cent of registered marriages involved women who had at least one child (under the age of 16) from a previous marriage (ABS 1992). However,

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since not all of these women will have children within their current marriage, the trend is likely to be affected only marginally. Secondly, and of more serious concern, is the treatment (or lack thereof) of ex-nuptial births - that is, births occurring outside of a legal marriage. In 1992, ex-nuptial births accounted for over 24 per cent of all births. Among women under the age of 20 (the majority of whom are not likely to have had more than one birth), over 84 per cent of total confinements in 1992 were ex-nuptial (ABS 1993). Unfortunately, it is not possible to present an extended time series of total first births by age of mother owing to limitations of the source data classifications.

As the research reported above suggests, changes in first birth timing have been examined using several measures. Delayed childbearing has often been measured by the lengthening interval between marriage and the birth (or conception) of the first child (Ruzicka 1976; Wilkie 1981; Teachman and Polonko 1985). Although this approach isolates the effect of childbearing delay from that of marriage postponement, the measurement of first birth delay from date of marriage has been criticized since it ignores the fact that the decision to postpone marriage is inextricably linked to the decision to delay the first birth (Rindfuss and St John 1983; Bloom 1984). This treatment also ignores childbearing outside of legally defined marriage which, as Glezer (1993:18) shows, is increasing in significance. Thus a relative, rather than absolute, measure of timing of first birth (that is, the proportion of women entering parenthood at given ages) will be employed here.

Demographic change can be assessed on two broad temporal dimensions - historical time and individual time. Change based on historical time can be measured using period data (in the present case, first births in each calendar year) or cohort data (birth cohorts of mothers). Individual time uses dis- aggregated information to assess change based on, in this application, age of mother at first birth. The use of period data alone has been criticized for providing only a cross-sectional picture of fertility involving many generations (see Ram 1990:151) which ignores the heterogeneity in fertility largely because of the limitations of official statistics (see Tuma and Huinink 1990:147). This paper will supplement the period analysis presented above by examining aggregate change using cohort data since, it has been argued, both approaches have independent merit (Bracher et al 1993). Individual level data will be used to assess the determinants of first birth timing and to examine the changing importance of factors over time.

The use of cohort or individual level data to examine changes in first birth timing is complicated since both are potentially subject to censoring - that is, the youngest cohorts or individuals may not have experienced their first birth at the time of investigation. Traditional data analytical techniques have not been able to handle censored data effectively, and these cases are frequently excluded from analysis. Such an approach is likely to substantially under- estimate the level of delayed childbearing since first birth postponement is a relatively recent phenomenon. Analysis of incomplete cohorts can be overcome, however, using life table survival analysis, and this method will be used to examine possible cohort changes in first birth timing.

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Analysis of individual level change is attractive since, given appropriate data, it allows for the investigation of a variety of socio-economic and other differentials in first birth timing. However, the use of survival analysis of individual level data substantially limits the number of simultaneous covariates of first birth timing to only a few variables, since the calculation of separate life tables for each combination of categories of interest is possible only in very large populations (Menken et al 1981).

Two further procedures have been developed and adapted to examine the covariates of age at first birth using data from incomplete cohort experiences. The first of these is based on the Coale-McNeil Model Nuptiality Schedule - a mathematical model used to estimate the proportion of women who will ever marry and the age at which first marriage is likely to occur - and has been subsequently adapted to the analysis of age at first birth (Rodriguez and Trussell 1980; Bloom 1982a, 1982b; Trussell and Bloom 1983). However, the failure of the Coale-McNeil Model Nuptiality Schedule to provide an adequate fit to observed first birth data has been demonstrated in several studies (Casterline and Trussell 1980; Hobcraft and Trussell 1980, cited in Bloom 1982a) which cautions against its use in this area. The second is based on Cox's proportional hazards model (Cox 1972) which has been used extensively to analyse truncated or censored data. Originally used in the field of medical research (Tibshirani 1982), proportional hazard modelling is gaining increasing use among social scientists (Menken et al 1981; Turna and Huinink 1990). As a result, a hazard rate regression model will be used in the multivariate treatment of covariates of postponed parenthood.

Data Sources Vital statistics data are supplemented by the use of the Australian National

University's Australian Family Project (AFP) Women's Survey 1986. The AFP Women's survey was a 1 -in- 1000 household survey of private dwellings in Australia based on a four-stage, area stratified probability sample. The overall sample coverage was 94 per cent of the Australian population, excluding sparsely populated rural areas and residents of non- private dwellings. Women aged between 20 and 59 years were considered eligible for interview. Face-to-face interviews were employed to collect information from some 2,547 women, or 79 per cent of those identified as eligible. The survey contains a range of fertility and demographic data on the life histories of respondents, as well as a substantial amount of information on their residential, educational, employment and marital histories (including the timing of significant demographic events), facilitating the analysis of differential aspects of first birth timing. Further details of the Project survey can be found in Bracher (1987).

Cohort Comparisons of First Birth Timing First birth survival functions for pairs of successive birth cohorts (not

shown here) indicate that inter-cohort differences are minor. In fact, between 1925-29 and 1950-54, no significant differences in first birth timing are evident. The only significant change in first birth timing among successive

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cohorts occurred between the 1950-54 and 1955-59 birth cohorts. Since women born after 1955 would enter their childbearing years around the early 1970s, this finding is consistent with the overall trend to delayed childbearing evident in the vital statistics data.

Differentials in First Birth Timing Despite the consistency of past research in identifying changes in first birth

timing, it is clear that the postponement of childbearing is by no means a universal trend. Bloom and Trussell (1984:608) have demonstrated that cohort fertility patterns in the United States are becoming increasingly hetero- genous, with recent cohorts showing much greater differences in timing and incidence of first births than do older cohorts. This finding is mirrored among German women, particularly those cohorts born after 1950 (Tuma and Huinink 1990:167).

A number of studies have sought to examine the differentials in first birth timing among (predominantly American) women and have, in the main, found that those most likely to delay childbearing are white, highly educated women with high levels of workforce participation, primarily in professional or white collar occupations, prior to embarking on their reproductive careers (Wilkie 1981; Rindfiiss and St John 1983; Teachman and Polonko 1985; Ram 1990). The impact of educational attainment on first birth timing in

Figure 2 Probability of remaining childless at each age by highest level of education

Source: Australian Family Project 1986, Women's Survey, unit record file.

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Australia is clearly demonstrated in Figure 2. While differences between primary and secondary educated women are minor, tertiary educated women are significantly more likely to delay the first birth than all other women. These observed differentials have led investigators to attribute the trend to delayed childbearing to the changing role of women - as indicated by higher educational attainment, growth in women's employment opportunities, and increased female participation in the labour force (Ram 1990: 147).

Explanations of First Birth Timing In an attempt to synthesize the diverse, yet overlapping, explanations of

postponed parenthood, Bloom (1984:115-121) has developed a conceptual scheme for analysing maternal age at first birth. The framework acknowledges the importance of four sets of interrelated factors in the explanation of first birth timing - social, economic, biological and stochastic.

Social Factors

There is little doubt that social factors have received more attention in past studies of first birth timing. In his framework, Bloom defines social factors in terms of preferences for particular lifestyle choices and norms about the role of women in relation to childbearing and alternative lifestyles. This set of factors, however, is quite extensive and requires further elaboration.

It is, perhaps, useful to consider normative influences on first birth timing as two sub-sets of social factors. Firstly, global, attitudinal changes such as increasing secularization and more liberal attitudes to the use of contraception and abortion have reduced the number of unwanted conceptions and births (Wilkie 1981:584). However, Rindfuss and St John (1983:562) have found that Catholics tend to have earlier first births than non-Catholics, and argue that this is a result of the doctrine's prohibition of contraception. The process of secularization would, one may expect, act to diminish this differential over time.

Secondly, changes in prevailing social mores work to legitimize different lifestyle choices for women - for example, careerism (centred on career advancement and upward social mobility), consumerism (oriented toward the enjoyment of material benefits and entertainment amenities), and familism (characterized by a desire for childbearing and a focus toward family life) (Bell 1958, 1968, cited in Knox 1990:183-184). Careerism as a lifestyle choice is hypothesized to result in increasing incidence of childbearing delay, and is often operationalized by measures such as highest completed level of education, workforce participation prior to childbearing, and occupation (Wilkie 1981; Rindfuss and St John 1983; Teachman and Polonko 1985; Ram 1990). Bloom, for example, states that studies which show a positive association between education and age at first birth provide evidence of 'increasingly ambitious education and career goals of women' (Bloom 1982a:367).

In addition to changes in norms and attitudes, structural factors, which are social in nature but have been ignored by Bloom, also influence first birth timing. These factors, such as opportunities for women to participate in

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education or the labour force (as opposed to preferences for such participation), may or may not be considered a result of normative changes, but need to be identified. For example, it has been suggested that women raised in rural settings may be less likely to postpone childbearing, since the lack of educational and career opportunities in the country may make motherhood a more attractive (or seem the only) available life trajectory (Rindfiiss and St John 1983).

However, it should be recalled that age at first birth was relatively high prior to the 1950s primarily due to the postponement of marriage and births during the depression and war years (Hugo 1986:44). Moreover, the increased age at first birth following the baby boom period was, in part, a return to more 'traditional' first birth timing. This pattern of childbearing delay indicates that more progressive attitudes to the role of women and advances in contraceptive techniques can only partially explain first birth timing.

Economic Factors Bloom's framework of first birth timing incorporates the importance of

economic factors, which he defines as the financial constraints which impinge upon or result from individual lifestyle choices (Bloom 1984:116). In addition to the direct costs of feeding, clothing and educating children, these financial constraints may also involve consideration of the opportunity costs of having children.

According to Bloom (1984: 116) women increasingly are becoming aware that disrupted labour force participation can lead to loss of skills and ultimately to decreased career advancement, both of which can have long term detrimental effects on household income. Wilkie (1981:585) has identified that, in fact, employment patterns of women have changed from a sequential work-mother-work cycle to continuous employment, establishing a career path prior to embarking on childbearing. Economic factors can also affect access to education and, hence, to lifestyle choices. Rindfuss and St John (1983:555), for example, suggest that women from large families may be restricted from participating in higher education due to the family's financial constraints, and therefore may not be in a position to choose a career-oriented lifestyle.

Bloom's framework can subsume complementing theories about the role of macro-economic factors in relation to first birth timing. Butz and Ward suggest that delayed childbearing results from structural change in the United States economy which provides greater incentives for women to work now than it did previously (cited in Bloom 1984:120). There is, in other words, a tendency for women to plan to have children during periods of low wages. Easterlin proposes that childbirth delay is a result of the effect of cohort size on employment opportunities. Competition for employment due to an over- supply of labour results in lower household incomes and parenthood is postponed to facilitate consumption of other goods and services (cited in Bloom 1984: 120). Indeed, it is clear that economic hardship resulting from the Great Depression had a significant impact on first birth delay, while periods of

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post-war economic prosperity coincided with earlier first birth timing (Teach- man and Polonko 1985:868).

Biological Factors Bloom further identifies the reciprocal causation of biological factors and

first birth timing. Aside from issues of infertility, the ability to plan the timing of the first birth is affected by the limited time span of a woman's fecundity and the known health risks associated with pregnancies in the latter stages of her reproductive span. Childbirth delay, however, can lead to infertility through endometriosis. Rindfuss and St John (1983:556) identify the inherent difficulties of measuring fecundity. As an indicator, they use the number of miscarriages and still births before the first birth.

Stochastic Factors

Finally, chance events may either act to promote or prevent the occurrence of a first birth at a particular age. Among these events, Bloom identifies the failure of contraception as well as the probability of conception itself and, in addition, processes which affect exposure to the risk of pregnancy, such as the age at which a woman meets her partner (Bloom 1984: 1 17).

Bloom acknowledges that the social, economic, biological and 'chance' factors incorporated by his framework are interdependent, and cannot be treated in isolation. Accordingly:

... women are viewed as decision makers with respect to the age at which they wish to bear their first child. They make this decision on the basis of their tastes and preferences for childbearing which they consider in the light of the various economic and biological constraints they face ... Although a woman may decide upon the age at which she desires to bear her first child, and although she may take certain actions designed to increase the chances of fulfilling that desire, the outcome of the first birth process is not determined entirely as a result of such individual choice and behaviour ... Stochastic factors ... may result in women having their first children before or after their desired ages (Bloom 1984: 118). Recent studies, however, have failed to take these multiple factors into

account when analysing first birth timing. There remains a serious need to examine the interaction between economic related period effects (such as unemployment levels and consumer and housing affordability) and social factors (such as differential values and life choices) on the timing of the first birth. Of course, any analysis of social science phenomena is limited by the inability of the researcher 'to deal with all variables in a social system, to measure and plot their interactions' (Miller 1991:286). Nevertheless, the adaptation of Bloom's framework for empirical analysis of first birth timing has considerable appeal, since it recognizes the multivariate influence of factors affecting first birth timing. In practice, however, the specification of these interrelationships is complicated by the difficulty involved in operation- alizing and measuring concepts of change associated with norms, attitudes and 'structural factors' (for example, 'careerism') thought to affect trends in delayed childbearing, as well as in the potential number of interaction terms required for a fully specified model that purports to explain these trends.

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It is thus beyond the scope of this paper to provide more precise measures of the concepts employed in the following analysis, or to examine all plausible interaction effects that might occur. Nor wiU any attempt be made to assess the indirect effects of factors through intermediate variables, or the reciprocal causation of proposed determinants. With this in mind, the following multivariate analysis incorporates a number of socio-economic and other measures which are related to processes identified under Bloom's framework, and are presumed to affect first birth timing of Australian women. Using a proportional hazards regression model, the net effects of individual influences on delayed childbearing are examined.

Proportional Hazards Regression Model of Delayed Childbearing

Religion, church attendance at age 14 and the use of contraception before the first birth are included since they are associated, in part, to changing attitudes to abortion and contraceptive use. Socio-economic indicators such as education, labour force participation prior to the first birth, father's occupation, residential location and size of family of origin are used since, as discussed earlier, they have been shown to affect individual lifestyle choices based on either social or related economic considerations. Biological factors are incorporated by measuring the number of foetal losses experienced prior to the first birth, while stochastic factors are controlled for by using an indicator of whether the conception of the first birth was planned or occurred by chance. Categorical variables which have more than two discrete categories (such as father's occupation) have been converted to dichotomous variables, one of which is excluded from the model and therefore acts as the reference category. Operational definitions of the variables incorporated in the model are provided in Table 1.

The introduction of global economic controls into the equation is problematic for a number of reasons. There are numerous indicators of consumer affordability and general economic climate, making it difficult to select an appropriate indicator (or combination of indicators). At the national level, indicators of the prevailing economic climate are based on: national output, income and spending; employment, unemployment and wages; production and business activity; prices; money, credit and security markets; and federal finance (Miller 1991:395). These indicators provide summary measures of the overall national economic situation but ignore individual level variations in women's financial resources. Some measure of individual financial status would overcome this problem but such measures are not available in the otherwise rich data source of the AFP survey. Even if they were, however, it is not clear at what point in a woman's life these indicators are likely to affect the decision to begin or delay childbearing. Further, such financial measures are subject to lag effects that cannot be easily predicted or measured. In recognition of these difficulties, the following analysis does not explicitly analyse global economic conditions that may impinge on the childbearing decision. However, it is assumed that the socio-economic variables specified will indirectly tap individual influences of material well-being.

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Table 1 Measurement of factors affecting first birth timing used in the proportional hazards regression model

XT . .- w Total N or N of

XT Vanablc . .-

Measurement w category coded (1)

Religion (0) Non-Catholic (1) Catholic 770 Church attendance at age 14

Never Excluded as reference category 423 Occasional (0)No (l)Yes 1,557 Frequent (at least once a week) (0)No (l)Yes 567

Size of family of origin Number of children 2,466 Father's occupation at age 14

Semi-skilled/unskilled (0)No (l)Yes 684 Skilled/clerical Excluded as reference category 744 Professional/managerial (0)No (l)Yes 847

Residential location at age 14 Capital city Excluded as reference category 1 ,042 Other major urban (0)No (l)Yes 413 Rural (0)No (l)Yes 1,092

Education prior to first birth Full-time equivalent in years 2,547

Employment prior to first birth Full-time equivalent in years 2,47 1

Contraceptive use prior to first birth (0) No ( 1 ) Yes 1,117 Foetal losses prior to first birth Number 2,547 Chance or planned conception of /mol /nnL n0%A first birth

(0) /mol Planned (1) /nnL Chance n0%A 734

In order to examine whether the effects of these factors change over time, separate regression models were conducted for each five-year birth cohort since 1925-30. Selected birth cohort results representative of the general pattern of influences over time are presented. Results of both the overall and time specific proportional hazards regressions are contained in Table 2. The risk ratios represent the relative likelihood of experiencing a first birth at each age, controlling for all other variables specified in the model. A risk ratio of one, for example, indicates that there is no difference in the survival curves, and hence in first birth timing, between the categories of discrete variables, or between units of continuous variables, under investigation. A risk ratio greater than one indicates a positive relationship between the age at first birth and the variable in question, while a risk ratio less that one indicates a negative relationship. The size of the risk ratio relative to unity indicates the percentage likelihood of experiencing a first birth at each age, given the individual has not yet experienced a birth. Only statistically significant results are reported here.

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Among all women in the survey, Catholics are 16 per cent more likely to have a delayed first birth than non-Catholics, assuming all other effects in the model are held constant. This may be due to later marriage. When examined over time, however, the importance of religion as a predictor variable is seen to decrease, since its only significant effect occurs among women born 1930-34. Among this cohort, the risk of having a first birth at each age, net of all other specified effects, was some 60 per cent greater for non-Catholics. Except for the youngest cohort, the effect of church attendance also decreases over time. Net significant effects of occasional church attendance, as opposed to no attendance, occur only among women born prior to 1950. Women who attended church services at least once a week when they were aged 14 are 17 per cent less likely to experience a first birth earlier than those who never went to church at the same age. This effect may occur as a result of that latter group marrying at a younger age, and thus being exposed earlier to the risk of childbearing.

When other variables are held constant, women whose fathers were in professional or managerial occupations were at greater risk of experiencing a first birth earlier than women whose fathers worked in skilled or clerical positions. This effect, however, was significant only among women born prior to 1950. Despite theories about the impact of residential location on lifestyle choice, no significant effects from rural upbringing are found in this analysis. Similarly, size of family of origin has only a marginal effect on first birth timing among all women, and among the cohort of women born between 1950-54.

For women born prior to 1940, education does not have a significant effect on first birth timing. However, its effect increased over time and was strongest among the youngest cohort of women - that is, those born during 1960-66. For these women, each subsequent year of full-time education decreases the risk of having a first birth at each age by some 13 per cent.

Full-time labour force participation does not have a substantial effect on first birth timing. Among all women its effect is negligible, and its only significant effects are found among those born between 1940-44, for whom each additional year of employment decreases the risk of having a child by three per cent, and between 1960-66, for whom each additional year increases the risk of childbearing by 13 per cent. This apparent inconsistency may be due in part to the changing nature of women's labour force participation, and increasing expectations among younger women of working until the birth of a child.

Foetal losses before the first birth have a significant effect overall, with each loss decreasing the risk of childbearing at each age by 14 per cent. Examined over time, however, foetal losses as a determinant of first birth timing are significant only for women born prior to 1935. Among all women, planned conception increases the risk of a first birth by approximately 170 per cent. The effect of this factor has generally been increasing over time and women born during 1960-66 who did not plan to conceive were 450 per cent less likely to have a first birth at all ages than those who planned conception. Since the first birth survival curves for women who planned their first birth and for women who did not are diverging, planned pregnancies are occurring

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increasingly at later ages - consistent with the finding of childbearing postponement.

This analysis suggests that determinants of first birth timing have, in fact, changed over time. Among early cohorts of women, factors which are hypothesized to influence attitudes to abortion and contraception, as well as biological and chance factors, were strong predictors of differential first birth timing. From the late 1950s and early 1960s, when women born between 1940-44 entered their childbearing years, the impact of more years of education began to appear. While education may also affect attitudes to abortion and contraception, its impact on first birth timing, along with workforce participation, is assumed to be more reflective of economic considerations and changing attitudes to the role of women - and, hence, of increasing variation in lifestyle choices of women. Difficult as it may be to dis- entangle these socio-economic influences on first birth timing, enhanced educational opportunities appear to have expanded women's control over the timing of their childbearing.

Conclusion The ability to identify trends and predict future incidences in childbearing

postponement, be it universal or differential, can greatly facilitate the social planning process. As noted previously, long term delayed childbearing will result in a decline in overall fertility levels - women who delay tend to have smaller completed family size, and later births decrease population growth by lengthening the time until the next generation reaches childbearing age. In the short term, however, postponement will create a lumpy effect, sending shock waves through the population age structure. As with the baby boom cohort, the identification of such irregularities in population composition can lead to more effective planning for social services such as child care places and schools (Wilkie 1981).

This paper has demonstrated that delayed childbearing is a phenomenon which has characterized the recent pattern of fertility both in Australia and in other Western countries. The timing at which women enter into the childbearing phase of their lives - and indeed whether they enter at all - has increasingly become a product of choice, facilitated both by normative societal changes and more effective measures of contraception. Decisions related to first birth timing seem to be based increasingly on both economic considerations and lifestyle choices, while random factors impose limits on the extent to which transition to childbearing is subject to individual preferences.

Acknowledgments The author gratefully acknowledges the assistance of Don Rowland,

Vance Merrill and Peter McDonald at the Australian National University in providing comments on earlier drafts of this paper, and the Social Science Data Archives at the Australian National University in providing access to the

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Australian Family Project 1986 Women's Survey1 unit record file and associated documentation.

1 Principal Investigators Gordon Alexander Carmichael and Michael D. Bracher, The Australian Family Project, Research School of Social Sciences, The Australian National University, 1987: Canberra. Data collected by McNair Anderson Associates.

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