Early weight gain adequacy may be a particular problem among adolescent gravidas because of their tendency to poor prepregnant nutritional status, nutritionally poor diets during pregnancy, and body image concerns, but it is not known whether irregular patterns of weight gain during adolescent pregnancy are associated with birth weight and length of gestation. We studied weight gain during pregnancy and pregnancy outcome in a cohort of 1790 teenage gravidas from Camden, New Jersey. We found that early inadequate weight gain (less than 4.3 kg by 24 weeks' gestation) was associated with a significantly increased risk of having a small for gestational age infant (adjusted odds ratio 1.88; 95% confidence interval 1.08-3.27), even when later gains brought the cumulative total weight gain to within adult standards. Late inadequate gains (less than 400 g/week) were associated significantly with preterm delivery (before 37 completed weeks' gestation), whether or not the total gain was adequate for gestation (adjusted odds ratio 1.69; 95% confidence interval 1.12-2.55). These results suggest that supplementation, intervention, or prenatal care protocols for adolescents that do not focus on balanced weight gain during adolescent pregnancy may reduce preterm delivery but may not significantly affect the incidence of intrauterine growth retardation.