Causes of perinatal mortality excess in prolonged gestations.

The study sought to determine what proportion of the perinatal mortality excess associated with prolonged gestations was due to placental insufficiency. Using data from a large prospective study of pregnancy, the perinatal mortality rate was 20.9/1000 births for post-term and 11.7/1000 for term infants. Only a quarter of the perinatal mortality rate excess in the post-term pregnancies was due to disorders related to inadequate uteroplacental perfusion, i.e., abruptio placentae, large placental infarcts and marked placental growth retardation. Twenty-six per cent of the mortality excess was due to congenital malformations, 19% to amniotic fluid infections, 8% to Rh erythroblastosis fetalis and the remaining 22% to a variety of other disorders. The post-term mortality excess due to congenital malformations was in infants who had hypoplastic adrenal glands, a well-known cause of prolonged gestation. The 4205 placentas of the post-term infants did not show any significant increases in those microscopic lesions that are characteristic of uteroplacental under-perfusion.