Themes and Variations

Abstract This chapter addresses particular challenges that epidemiologists encounter in the study of fertility and pregnancy. Denominators are often missing for specific outcomes such as birth defects, which limits causal interpretation. Reproductive events can compete with one another—for example, a pregnancy ending in miscarriage is unable to contribute to preterm risk. A short interval between exposure and outcome in the study of pregnancy outcomes can be an advantage for data collection but also opens the door to reverse causation. Extremely long intervals between exposure and outcomes are also possible, as with the link between fetal exposure to diethylstilbestrol and later breast cancer. Privacy is an ongoing issue. The processes of fertility and pregnancy involve personal information and decisions that participants may not wish to share.