Fertility Postponement and Apparent Fecundability. Analysis of Relation with Use of Retrospective Survey Data.

Krzysztof Tymicki, Demography Unit, Warsaw School of Economics

Paper analyses relationship between advances in age at first attempt to conceive and apparent fecundability defined as detection of a pregnancy. Analysis focuses on biological consequences of age shifts in fertility schedules in contemporary low fertility societies. Due to age specific decrease in biological ability for natural conception, women may experience increase in waiting time to pregnancy measured as interval between onset of deliberate attempt to conceive and pregnancy detection. Fecundability data come from two waves of Polish GGS survey collected with a short retrospective questionnaire. Using hazard regression model we have found significant increase in waiting time to pregnancy among women aged 35+, slight increase among females aged 30-35 and no effect for younger females. Results are in line with other studies which suggest that postponement of reproduction beyond age of 35 might influence probability of conception, transition to higher parities and childlessness. Taking into account current fertility patterns, age-specific decrease in apparent fecundability might have only moderate effect on observed fertility rates and realization of fertility intentions. However, systematic increase in age at first effort to conceive might be potentially a factor in a reproductive health especially among social groups which are in the avant-garde of fertility postponement.

The analysis focuses on biological limits to fertility postponement in the context of female fecundity during life cycle. This research question is important in the context of observed age at birth of the first child, which is driven by postponement of decision concerning onset of deliberate attempts to conceive.

Opposing trends of decrease in fecundity and simultaneous increase in age at first attempt to conceive we expect to observe difficulties in realization of fertility intentions resulting in lower than planned number of offspring, more difficult progression to parity two and higher and finally increase in the share of childless. The purpose of analysis is to find out whether females in contemporary developed societies have reached age at first attempt to conceive where decrease in fecundity might seriously endanger waiting time to first pregnancy, progression to higher order births and childlessness.

Most of demographic analyses focuses on socio-economic factors affecting fertility decision such as situation on labor market, education or institutional context. These analyses seems to ignore increasing divergence between life-cycle and changes in female fecundity. For instance, most of demographers assume that births lost due to postponement are possible to regain as a result of recuperation effect. This implicitly assumes constant pattern of female fecundity over age or at least that decision concerning pregnancy is not delayed beyond the point of significant loss of reproductive potential. From the perspective of studies on age patterns of female fecundity contemporary developed societies have reached age at first attempt to conceive where apparent fecundability stands for approximately 70% of maximum value.

Reproductive ageing has been under study for a long time but apparently has not been incorporated extensively into demographic studies. First approaches to study variability of human reproductive potential over age also called reproductive ageing come from the field of evolutionary ecology of human reproduction. A significant increase in life expectancy, which resulted from social and economic development along with diffusion of contraception, changed relation between evolutionary shaped fecundity and individual decisions concerning procreation. It seems that humans have not been evolutionary designed for late procreation as it is observed in contemporary developed societies. Average age at first procreation in historical European societies almost overlap with age of maximum fecundity which allowed females to achieve much higher completed fertility even in highly unfavorable mortality conditions and no contraception. Taking into account increase in life expectancy, decrease in infant mortality rate and overall drop in quantum of reproduction decreasing fecundity might have no effect on realization of fertility intentions. However influence of reproductive ageing with respect to realization of reproductive intentions might again gain its significance as average age at procreation reached age 30 and beyond. Late age at onset of procreation might result not only in increase of sub-fecundity or infertility but also in less time for medical intervention or application of assisted reproductive technologies.

Analysis accounts for impact of reproductive ageing on chances of successful procreation by measuring waiting time to pregnancy (WtP). This simple retrospective methodology reflects generally approved definitions of sub-fertility and infecundity as number of months with regular unprotected intercourse ending (or not) in pregnancy. Measurement of WtP allows estimate proportion of sub-fertile and sterile couples as well as account for changes in conception probability with age. Therefore, postponement of reproduction and simultaneous decrease of fecundity might be responsible not only for increase in waiting time to first pregnancy but also in increase in proportion of sub-fertile and childless couples.

Measurement of WtP has been used in both waves of Polish GGS. We have asked questions concerning infertility and questions concerning number of menstrual cycles needed for conception. This retrospective method allowed us measurement of waiting time to pregnancy with respect to age of respondents. Analyses reveal that there is a significant increase in WtP among women aged 35+ and slight increase in WtP among females aged 30-35. At this same time we observe no differences in WtP among younger females. Although we observe, as predicted, the effects of reproductive ageing for females 35+ taking into account age at first birth in Poland (around 29 years in 2012) the effect might not influence completed fertility rates on the level of population in a significant way. Although we may think that within some social groups characterized by more substantial postponement of fertility this effect could be more pronounced.

We conclude that decrease in fecundity might have a particularly strong effect for women who have reached age of 35. Beyond that age fecundity drops to about 60% of maximum value for women aged 20-22. In such case the interval between onset of deliberate attempt to conceive and actual pregnancy might get wider presumably increasing chance for being childless or in case of successful first pregnancy it might reduce chances for second pregnancy.

Presented in Session 1234: Sexual and Reproductive Behaviour