Henri Leridon

Many factors act simultaneously in childhood to influence health status, life chances and well being, including pre-birth influences, the environmental pollutants of early life, health status but also the social influences of family and school. A cohort study is needed to disentangle these influences and explore attribution.

Over the past decades, the proportion of couples who resort to infertility treatment has tremendously increased, and fertility (the final number of children) sharply declined. We explored the roles of two potential causes of these trends: a temporal decline in the couples' fecundability and a postponement of age at initiation of pregnancy attempts. We conducted a Monte–Carlo simulation for the reproductive history of 100 000 women based on the fertility and socio-demographic characteristics of the 1968 birth cohort in France. Declines in fecundability of various amplitudes have been implemented, as well as increases in the distribution of age at initiation of pregnancy attempts. A decline in fecundability by 15% implied a decrease in fertility by 4%, and an increase in the proportion of couples eligible for infertility treatments by 73%. An increase in the mean age at initiation of first pregnancy attempt by 2.5 years from 25 years entailed a decrease by 5% in fertility and an increase by 32% in the proportion of couples eligible for infertility treatments. A relatively important decrease in fecundability and an increase by 2.5 years in age at first pregnancy attempt are likely to have only a limited impact on fertility. However, they may have a large impact on the proportion of involuntarily infertile couples, likely to resort to assisted reproduction techniques.

The proportion of couples permanently sterile beyond a certain age is an important component of the reproductive process. Unless medical assistance is used, this age is the upper bound of the fecund period. Most estimates of sterility by age of the woman have been derived from natural fertility populations, in which the number of births and the timing of the last birth (of the complete reproductive history) were not controlled by the couples. Because data on these populations do not include pregnancies not ending in a live birth, the sterility estimates apply to the proportion of couples unable to conceive and to have a live birth. For this reason, it is useful to have an estimate of sterility based on the risk of conceiving, independently of the fate of the pregnancy. Using this new estimate, sterility increases with age much more slowly than with most previous estimates.

The approaches used by demographers and by epidemiologists for studying the fecundity of couples (i.e. their ability to conceive) have converged, whereas they were historically divergent. Demography traditionally focused on fertility – the circumstances in which livebirths occur – with the aim of predicting population changes, in particular. Demographers also rapidly became interested in fecundity, developing concepts such as fecundability and definitive sterility . The measurement of fecundability is based on determination of the time to pregnancy (TTP), the basic tool of the epidemiologist. However, while demographers were developing methods for estimating the distribution of fecundability among couples based on TTP, epidemiologists turned to methods for analysing the role of diverse factors potentially influencing fecundity at the individual level.

The use of the contraceptive pill increased very rapidly in the 1970s in many developed countries, and fertility almost simultaneously started to decline. We discuss here the possibility of a causal link between these two major changes. We first provide evidence for a relationship between the spread of oral contraceptive use and the change in fertility in many European countries over the last three or four decades. The situation of specific countries is examined more in depth on the basis of available literature. We then review the various theories attempting to explain these trends and see how the family planning variables are treated in these approaches. At the country level, the conclusion is unambiguous: within individual countries, there is no systematic negative correlation between fertility and contraceptive pill use. The development of hormonal contraception cannot be considered as responsible for either starting or the size of the fertility decline. A more subtle chain of causality must be considered, but there is no agreement on a general theory of fertility changes. Most authors, however, agree that the diffusion of modern contraception has certainly contributed to the reduction in the number of unwanted pregnancies and has also facilitated and favoured the adoption of new (more restrictive) norms for the ideal family size.

BACKGROUND: Maternal age is an important factor in reproduction. Can assisted reproduction technologies (ART) fully compensate for the decline in fertility with age? METHODS: We used a computer simulation (Monte Carlo) model of reproduction, combining the monthly probabilities of conceiving, the risk of miscarriage and the probability of becoming age‐dependently permanently sterile. RESULTS: Under natural conditions, 75% of women starting to try to conceive at age 30 years will have a conception ending in a live birth within 1 year, 66% at age 35 years and 44% at age 40 years. Within 4 years the success rates will be respectively 91, 84 and 64%. If women turn to ART after 4, 3 or 2 years respectively without conception, and if the rate of success is as observed after two cycles of insemination in IVF, ART makes up for only half of the births lost by postponing a first attempt of pregnancy from age 30 to 35 years, and <30% after postponing from 35 to 40 years. CONCLUSIONS: Even if we relax some of the assumptions, ART in its present form cannot make up for all births lost by the natural decline of fertility after age 35 years.