Many physicians believe that macrosomia is a hallmark of a pregnancy complicated by glucose intolerance. Because the prevalence of obesity is increased among women with gestational diabetes, fetal overgrowth may be attributable at least in part to maternal obesity. We studied 2069 black, Latina, Chinese, and white mother-infant pairs to determine the interaction between maternal body habitus, maternal glucose homeostasis, and certain indices of fetal growth. Chinese women had a significantly higher serum glucose 1 hour after administration of 50 g glucose (136.6 +/- 32.7 mg/dL) than any of the other three ethnic groups. Black women had a significantly lower value for glucose (114.8 +/- 28.2 mg/dL) than either Chinese or Latina women (124.9 +/- 31.4 mg/dL). Results for Latina and white women (121.5 +/- 26.2 mg/dL) were not significantly different. Body mass index (BMI) was used to classify the subjects. The regression coefficient for the entire sample indicated a modest association of glucose with increased birth weight when maternal BMI was controlled. The BMI of the Chinese infants had a significant association with higher concentrations of glucose after administration of 50 g glucose. Maternal body habitus should be considered a major confounder in studies of the relationship of maternal glucose tolerance and infant birth weight.