Recurrence of gestational age in sibships: Implications for perinatal mortality

The authors studied the extent to which preterm birth and perinatal mortali ty are dependent on the gestational ages of previous births within sibships . The study was based on data collected by the Medical Birth Registry of No rway from 1967 to 1995. Newborns were linked to their mothers through Norwa y's unique personal identification number, yielding 429,554 pairs of mother s and first and second singleton newborns with gestational ages of 22-46 we eks, based on menstrual dates. Siblings' gestational ages were significantl y correlated (r = 0.26). The risk of having a preterm second birth was near ly 10 times higher among mothers whose firstborn child had been delivered b efore 32 weeks' gestation than among mothers whose first child had been bor n at 40 weeks. However, perinatal mortality in preterm second births was si gnificantly higher among mothers whose first infant had been born at term, compared with mothers whose firstborn child was delivered at 32-37 weeks. S ince perinatal mortality among preterm infants is dependent on the gestatio nal age in the mother's previous birth, a common threshold of 39 weeks' ges tation for defining preterm birth as a risk factor for perinatal death may not be appropriate for all births to all mothers.