Epidemiologic approaches for studying recurrent pregnancy outcomes: challenges and implications for research.

The study of recurrence of pregnancy-related complications and outcomes can offer powerful insights to understanding patient-related risks for subsequent pregnancies. Such studies, when designed, analyzed, and interpreted correctly, can help distinguish genetic from environmental causes that portend increased recurrence of a particular pregnancy complication (eg, recurrence of gestational diabetes) or a perinatal outcome (eg, recurrence of preterm birth or preeclampsia). Recurrence risk studies can be challenging in other dimensions, including inherent biases, generalizability of findings, inadequate study size, and inappropriate use of analytic models to study recurrence. Other common misperceptions in studies of recurrence risk are highlighted, including issues with terminology and interpretation of recurrence risks. A review of available epidemiologic study designs is presented and the usefulness and applicability of each design for addressing specific etiologic questions as they relate to recurrence risks are contrasted.