Birth weight-length ratios, ponderal indexes, placental weights, and birth weight-placenta ratios in a large population.

BACKGROUND: Birth weight (BW)/length ratios, ponderal indexes, placental weights, and BW/placental weight ratios have been correlated with perinatal morbidity and mortality.

METHODS: Cross-sectional survey of live-born, structurally normal, singleton neonates delivered at a tertiary care center in Detroit, Mich, from 1984 through 1991. Subjects were included if dating based on menstrual dates and ultrasonographic estimations agreed with Ballard examination to within 2 weeks.

RESULTS: Of 33,138 neonates, 26,983 (81.4%) were black. While all indexes significantly increased with advancing gestational age, only placental weights (without prior draining of blood, blotting the placenta dry, or trimming the membranes or cord) and BW/length ratios significantly increased beyond 40 weeks' gestation. Multivariate analysis of variance revealed that placental weights, BW/placenta ratios, and BW/length ratios were significantly related to gender and ethnicity, but ponderal index was not. Placental weights were larger, and BW/placenta ratios were smaller than studies that trimmed the placentas before weighing.

CONCLUSIONS: All indexes were gestational age dependent, but ponderal index and BW/placenta ratios do not increase beyond 40 weeks' gestation. Except for ponderal index, interpretation of these indexes should be specific for ethnicity, gender, and manner of placental preparation.