We should monitor human fecundity, but how? A suggestion for a new method that may also be used to identify determinants of low fecundity.

Human fecundity may be declining, and we may need ways to monitor it. The most simple monitoring is based on measuring waiting time to pregnancy retrospectively among pregnant women. Unfortunately, this design does not provide an estimate of fecundity, because infertile couples are excluded. We suggest a novel approach that combines the measurement of waiting time for pregnant women with information on the distribution of waiting time among women waiting to become pregnant. The method is based on principles from the case-cohort design.