Pregnancy weight gains were examined at 4-week intervals from 12-36 weeks' gestation and total gain assessed at delivery in a cohort of 2008 pregnant women aged 18 or less at entry to prenatal care. As early as 12 weeks' gestation, there was a significant association between the amount of weight gained and infant birth weight measured at the time of delivery. At 16 weeks' gestation, gains below the 25th percentile were associated with an increased risk of low birth weight (LBW) (adjusted odds ratio 1.56; 95% confidence interval 1.01-2.43), and by 20 weeks' gestation, the risk of LBW was doubled (adjusted odds ratio 2.00; 95% confidence interval 1.34-2.99). Also at 16 weeks, there was a doubling in the risk of excessive fetal size or macrosomia (adjusted odds ratio 2.31; 95% confidence interval 1.31-4.10) associated with maternal weight gain above the 75th percentile. These results suggest that an increased risk of certain poor pregnancy outcomes is detectable late in the first or early in the second trimester. Consequently, weight gain monitoring may be important early in pregnancy.