Currently available formulas for the estimation of fetal weight assume uniform density of tissue. Because fat tissue is less dense than lean body mass, we hypothesized that the sonographic overestimation of fetal weight in infants of diabetic mothers is the consequence of an elevated proportion of body fat, resulting in a lower body density. We prospectively examined 52 children of diabetic mothers. Each had ultrasound estimation of fetal weight within 7 days of delivery and estimates of neonatal body composition made from anthropometric evaluation within 48 hours of birth. Ultrasound estimates of fetal weight were considered acceptable if they were within 10% of actual birth weight. There was no difference in mean birth weight between those overestimated (N = 22) and those underestimated (N = 8). The sum of skinfolds from two sites, the ponderal index, and percent body fat were all significantly greater in the neonates with sonographic overestimation of fetal weight. Lean body mass was significantly greater (P less than .05) in infants whose sonograms underestimated birth weight. When all subjects were included, a significant correlation was found between the degree of error in the ultrasound estimation of fetal weight and the ponderal index (r = 0.40, P less than .01), the sum of the skinfold measurements (r = 0.29, P less than .05), and the present body fat (r = 0.28, P less than .05). These data suggest that increased body fat in infants of diabetic mothers is associated with sonographic overestimation of fetal weight.