Paternal genes as expressed by the fetus play a role in the timing of birth and in the risk of repeating a prolonged pregnancy Prolonged pregnancy—a pregnancy with a gestational length of 294 days or more—occurs in about 5% of all births. It is associated with a higher frequency of obstetric complications and perinatal morbidity,1 and little is known about its aetiology.2 We studied the risk of recurrence of prolonged pregnancy as a function of change in partner or change in social conditions. We obtained data recorded in the Danish medical birth registry on all women with a prolonged pregnancy in the first delivery and in a subsequent delivery, during 1980-94, and a 5% sample of all women with two or more pregnancies recorded in the period 1980-92 (only the first two deliveries were used for analysis). The information on gestational age in the registry was obtained from birth records that had been …