Inadequate weight gain during pregnancy is an important risk factor for low birth weight (LBW), but the contribution of diet to weight gain is uncertain. Pregnancy weight gains were examined at 4-week intervals from 12 to 36 weeks' gestation, as well as total gain for gestation, in a cohort of over 2000 young pregnant women, aged less than or equal to 18 at entry to prenatal care. The effect of diet was studied in a 15% random sample of the cohort, using a 24-hour dietary recall obtained at entry to prenatal care (averaging 17 weeks' gestation). As early as 16 weeks' gestation, gains below the lower limit of a clinical standard were associated with a decrement in birth weight at delivery of more than -85 g (p less than 0.01), and after 24 weeks of approximately -180 g (p less than 0.001). After adjusting for potential confounding variables, teenagers who went on to develop inadequate total weight gain for gestation had consumed 1878 kcal vs 2232 for teenagers with adequate total gain (p less than 0.05). There were significant deficits in protein (p less than 0.05) and carbohydrate intake (p less than 0.05) associated with inadequate gain. However, there was no direct effect of nutrient intake on birth weight, LBW, or preterm delivery. This suggests that the relationship between nutrient intake during pregnancy and birth weight may be indirect and moderated by weight gain during pregnancy.