These data are a review of previously published data. Initially, body composition was estimated in 186 neonates. Fat- free mass (FFM), which constituted 86% of birth weight, accounted for 83% of the variance in birth weight; fat mass (FM), which constituted 14% of birth weight, accounted for 46% of the variance in birth weight. Male neonates were an average of 175 g heavier than females. FFM was greater among males compared with females (P = 0.0001). Using stepwise logistic regression, 29% of the variance in birth weight, 30% in FFM, and 17% in FM was accounted for. Independent variables included maternal height, pregravid weight, weight gain during pregnancy, education, parity, paternal height and weight, neonatal sex, and gestational age. Including maternal insulin sensitivity explained 48% of the variance in birth weight, 53% in FFM, and 46% in FM. There was a positive correlation between weight gain and birth weight in control subjects but a negative correlation in subjects with gestational diabetes mellitus. Lastly, the roles of insulin, insulin-like growth factors, and leptin were examined in relation to fetoplacental growth and body composition. The assessment of fetal/neonatal body composition may improve the understanding of the effect of differential factors on fetal growth. Factors associated with accretion of fetal adipose tissue in late gestation are less well understood compared with birth weight and FFM. Additional studies of maternal glucose and lipid metabolism are needed to better evaluate fetal growth.