Gestational duration and birthweight in white, black and mixed-race babies.

Using the 1983 United States population of single live births, birthweight and gestational duration were compared for babies of these different parental racial groups: both parents White, mother White-father Black, mother Black-father White, both parents Black. The four groups differed significantly with respect to the usual sociodemographic variables. Mean birthweight and mean gestational duration decreased in that order from the White-White reference group, and conversely there were increasing trends for low birthweight and preterm delivery. Adjustment for the usual sociodemographic variables did not alter these trends appreciably. Group differences were more strongly related to the mother's race than to the father's, and the trends were related to the mother's race. Because the father's race was significant, genetic factors are probably of some importance. The evidence from this study, together with the often-demonstrated relationships between low birthweight and preterm delivery with sociodemographic variables, and the short-term downward secular trends in low birthweight, support the concept that non-genetic maternal factors are more important for these abnormal outcomes. But because neither the usually utilised sociodemographic variables nor genetic factors seem to explain much of the group differences, new approaches are necessary to understand why, irrespective of ethnic group, some women are at excess risk for suboptimal birth outcome.