Evaluation of fetal growth by estimation of neonatal body composition.

To characterize the variation in normal fetal growth by body composition analysis, 188 neonates from uncomplicated singleton term pregnancies were evaluated within 24 hours of birth. Anthropometric measures used to estimate lean body mass and body fat included the following: birth weight 3553 +/- 462 g, lean body mass 3060 +/- 377 g (86.3%), fat mass 495 +/- 196 g (13.7%), and ponderal index 2.65 +/- 0.25. There was a significant linear correlation between birth weight and lean body mass (r2 = 0.83, P = .0001), fat mass (r2 = 0.46, P = .0001), and ponderal index (r2 = 0.22, P = .001). Although the ponderal index has been used as an index of corpulence, the correlation between ponderal index and percent body fat was poor (r2 = 0.15). These results suggest that although neonatal fat mass constitutes only 14% of total birth weight, it explains 46% of its variance. In contrast, the ponderal index explains only 22% of the variance in birth weight and correlates poorly with percent body fat. Body composition analysis explains a significant amount of the variance in normal birth weight.