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 …