INFLUENCE OF FETAL FAT ON THE ULTRASOUND ESTIMATION OF FETAL WEIGHT IN DIABETIC MOTHERS

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 fet al weight in infants of diabetic mothers is the consequence of an elev ated proportion of body fat, resulting in a lower body density. We pro spectively examined 52 children of diabetic mothers. Each had ultrasou nd estimation of fetal weight within 7 days of delivery and estimates of neonatal body composition made from anthropometric evaluation withi n 48 hours of birth. Ultrasound estimates of fetal weight were conside red acceptable if they were within 10% of actual birth weight. There w as no difference in mean birth weight between those overestimated (N = 22) and those underestimated (N = 8). The sum of skinfolds from two s ites, the ponderal index, and percent body fat were all significantly greater in the neonates with sonographic overestimation of fetal weigh t. Lean body mass was significantly greater (P < .05) in infants whose sonograms underestimated birth weight. When all subjects were include d, 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 < .01), the sum of the skinfold measurements (r = 0.29, P < .0 5), and the percent body fat (r = 0.28, P < .05). These data suggest t hat increased body fat in infants of diabetic mothers is associated wi th sonographic overestimation of fetal weight.