Trends in fetal growth among singleton gestations in the United States and Canada, 1985 through 1998.

We examined trends in fetal growth among singleton live births in the United States and Canada. The data files (n = 48,637,680; 16.6% blacks) for US births, and the Canadian Birth Database of Statistics Canada (n = 3,167,702) for Canadian births were used. Trends were assessed between 1985-86 and 1997-98 with reference to mean birthweight, birthweight-for-gestational-age z-score, and proportions delivered low birthweight (< 2,500 g), small for gestational age (SGA: birthweight < 10th centile for gestational age) and large for gestational age (LGA: birthweight > 90th centile). The term "mean birth weight" increased in the US and Canada between 1985 and 1998, as have the mean z-score. Rates of term SGA births declined among US (11% among whites and 12% among blacks) and Canadian births (27%). Preterm SGA births increased by 3% and 17%, respectively, among US whites and blacks, but declined by 11% among Canadian births. Further, term LGA births increased in the US (5% among whites and 9% among blacks) and Canada (24%). Preterm LGA births declined by 13%, 25%, and 14% among US whites and blacks, and Canadian births, respectively. These findings suggest that US and Canadian babies are getting bigger. The role of preterm obstetrical induction and preterm cesarean delivery are likely to have influenced these trends.