Birth weight standards for triplets under modern obstetric care in the United States, 1984-1989.

Birth data were reviewed on 3321 live-born infants from 1138 triplet pregnancies delivered in the United States between 1984-1989. The three major etiologies for the multiple gestations were fertility drugs (50%), spontaneous (38%), and in vitro methods (9%). The average length of gestation was 33.8 weeks and the mean birth weight was 1911 g. Neonatal birth weight curves for triplet infants born alive in the third trimester were plotted. From 26-35 weeks, the average triplet newborn has a weight corresponding to approximately the 30th percentile level compared with singletons. After 35 weeks, triplet birth weights fall progressively behind those of singletons, reaching the tenth percentile at 38 weeks. Multiple epidemiologic factors were analyzed to determine their effect upon neonatal birth weight and length of gestation. Factors predicting higher than average birth weight included male sex, increasing maternal age, increasing maternal height and weight, maternal weight gain, and maternal parity. The length of gestation was found to correlate with maternal age, weight gain, and parity. No significant association between fertility method and gestational age or weight could be identified. This large data base provides the first comprehensive percentile birth weight rankings for modernly managed triplet gestations in the United States population. A regression equation is presented which accurately predicts mean triplet birth weight in the third trimester and which suggests that a nearly linear weight gain of approximately 150 g per week per fetus should be expected in this period.