In 40,635 deliveries in 1978-85, unexplained stillbirths were an important component (nearly a quarter) of all perinatal deaths. The rate of unexplained stillbirth (unexplained stillbirths divided by total births) was highest among preterm deliveries, fell to a minimum at 39-40 weeks' gestation, then rose at 41-42 weeks. Rate is generally accepted as measuring risk, but since it is the population of undelivered, not delivered, infants that is at risk of intrauterine death, stillbirth risk would be better measured as the number of impending stillbirths divided by the total number of undelivered fetuses. With this measure the risk of unexplained stillbirth was least in preterm pregnancies, rising fourfold after 39 weeks to a maximum at 41 weeks. At this time, it was also four times higher than at 33 weeks, in contrast to the rate, which was nineteen times lower.