About 5% of babies are born postterm (that is, delivered after 42 completed weeks of gestation). Postterm infants experience more morbidity and mortality than term infants, prompting routine (and expensive) antenatal testing and active management of postterm pregnancies. This article reviews the epidemiology of postterm delivery. A few congenital conditions associated with disruption of the fetal-pituitary-adrenal axis as well as a rare maternal enzyme deficiency have long been identified with postterm delivery. In recent literature, environmental pollution, diet, and pharmaceutical agents have been associated with postterm birth. Very little systematic research has focused on identifying risk factors for this poorly understood birth outcome.