Interpreting trends in fecundity over time

Infertility is a common problem in affluent societies. It affects around 15% of couples trying to conceive, although not all seek medical help.1 In some countries, up to 6% of children are conceived through assisted reproductive techniques.2 Many young men have sperm counts that fall short of the limit known to be associated with reduced fecundity.3 It is still unclear whether the past decades have seen a substantial change in the fertility of couples in general or in the fertility of men in particular.4

The few studies that have examined changes in fecundity over time (or menstrual cycles)—from discontinuation of contraception to pregnancy—show conflicting results.5 Fertility in couples is determined by social, behavioural, and biological factors that cannot be reliably ascertained in studies based on retrospectively collected data. Therefore, we will never know if biological fertility has changed.5 Findings of numerous studies of secular trends in sperm counts are far from conclusive, but a study by Jensen’s group provides convincing evidence of a pronounced difference in semen quality between populations.6 Moreover, another recent paper from the same group shows that in cohorts of Danish women born between 1960 and 1980, birth rates decline progressively after adjustment for children conceived by assisted reproduction. The results are even more pronounced when the sharply declining rate of induced abortions is accounted for.7

However, biological fecundity is just one of the many determinants of fertility. These includes sexual behaviour, desire for a given family size, social conditions, the age at which people start to have children, use of family planning methods, and the availability of assisted reproductive techniques. These cultural and social norms may mask more subtle biological changes in the population. More direct markers of fecundity are urgently needed, and the time has probably come to include fecundity in ongoing representative health surveys. Drawing on the present understanding of the methodological pitfalls in fertility research, we may be able to collect prospective data that are sufficiently comparable over time. Such studies may detect changes in fecundity similar in size to those reported in the past.

In any case fecundity is expected to decline over time, even if no evident causative environmental exposures are present.8This is because fecundity probably has a strong genetic component. With the advent of assisted conception, subfertile couples may have as many children as fertile couples, so that genetic factors linked to infertility will become more prevalent in the generations to come.9

From a public health perspective research should focus on avoidable causes of subfecundity. Such studies should look at exposure from the time of development of the sexual organs in the fetus to the time of trying to become pregnant. The first time point may be at least as important as the second. Thus, promising new results show that intrauterine exposure deserves close attention, and that both lifestyle and environmental factors should be the focus of further studies.10 11 One study found a threefold higher prevalence of DNA damage in infertile men with Chlamydia trachomatis infection of the genitourinary tract than in fertile men without infection.12 Treatment with antibiotics reduced the signs of DNA damage, and the female partners of a small subset of patients became pregnant after the treatment ended. The cross sectional design of the study means the findings should be interpreted with caution, and further studies are needed to confirm the results.

The best way to counteract infertility and help couples to have children naturally is to deal with the avoidable causes of subfecundity. Disappointingly, a large new programme for establishing research centres in reproduction and infertility in the United States devoted little attention to the environmental causes of subfecundity. There are good grounds for promoting further research and for trying to make up for the many years during which research into infertility has been neglected. The endocrine disruption hypothesis—which states that environmental chemicals may cause adverse development of sexual organs by interference with hormonal regulation—is just one of many hypotheses that deserve attention from funding agencies.11