Pregnancies lasting longer than 293 days (42 weeks and beyond) are considered postterm, which carry greater risks for maternal and fetal complications. The etiology of postterm pregnancy is largely unknown, but women of older age, those with higher body mass index, those who are nulliparous, and those carrying a male fetus are at higher risk. This population-based study was undertaken to examine the familial clustering of postterm birth, to determine whether potential clustering could be explained by familial sharing of risk factors, and to quantify the genetic and environmental contributions to postterm birth.