SOURCES OF BIAS IN STUDIES OF TIME TO PREGNANCY

Among sexually active:couples who are not using contraception, there i s considerable heterogeneity in fertility, even among those who eventu ally achieve pregnancy. The number of menstrual cycles required, measu red in integer time, is greatly overdispersed compared to the geometri c, suggesting that the per cycle probability of conception varies cons iderably among couples. Some of this variability may reflect the effec t of reproductive toxicants on fertility, and studies of time to pregn ancy can be useful in identifying such toxic effects. We describe mode ls for analysing time-to-pregnancy data, and discuss seven sources of bias that can lead the reproductive epidemiologist to spurious conclus ions. Certain analytic and design strategies can help protect against some of the pitfalls,