Gestational Age and Preterm Delivery

Abstract Gestational age is arguably the most important measurement in reproductive epidemiology. Gestational-age criteria separate miscarriages from stillbirths, and preterm births from term births from post-term births. Gestational age is also a crucial variable in establishing a woman’s eligibility for elective abortion. Unfortunately, gestational age remains one of the most difficult variables to measure accurately. For the majority of pregnancies, the date of conception can be estimated only indirectly. This difficulty of measurement has consequences for virtually every aspect of pregnancy research. Widespread use of the last menstrual period (LMP) is both a practical solution for dating pregnancy and a source of error, bias, and confusion. To further muddy the waters, the strong associations of short gestation (preterm delivery) with adverse pregnancy outcomes reflect both causal mechanisms and confounding by unmeasured factors. Methodological problems abound.