Racial differences in pregnancy duration and its implications for perinatal care.

The average length of gestation is about 5 days shorter in black populations than in white populations. Although some of this difference is accounted for by higher preterm delivery rates in blacks, the most common gestational week of delivery at term is the 39th in black populations, the 40th in white. Black gestational age specific neonatal mortality is lower than that of whites until the 37th week of gestation, but higher thereafter. These observations suggest the hypothesis that complications of postmaturity occur sooner in black fetuses. If this hypothesis is confirmed, antepartum surveillance for signs of fetal compromise should be initiated earlier in gestation in black parturients, perhaps by setting the estimated date of confinement at 275 days after the LMP, rather than the conventional 280 days.