We examined the influence of two measures of maternal nutritional status: prepregnant body mass (kilograms/meter²) and Weight gain during pregnancy (adjusted for duration of gestation) on spontaneous preterm delivery (<37 completed Weeks' gestation) and duration of gestation, as well as on low birthweight (<2,500 grams) and small‐for‐gestational‐age (SGA) in pregnant adolescents. Inadequate Weight gain for gestation increased the risk of spontaneous preterm delivery when prematurity was reckoned by the obstetric estimate (Adjusted Odds Ratio, AOR = 1.75 95% CI 1.22–2.50) or from the mother's LMP (AOR = 1.49 95% CI 1.10–2.02). In linear models, gestation duration was significantly reduced, by more than half a week, when reckoned from eiter estimate of gestation. However, the association between preterm birth or gestation duration and prepregnancy body mass was not consistent and depended on the method of estimating gestation. Low birthweight and SGA were each significantly associated with inadequate weight gain during pregnancy as well as with prepregnant body mass. These results suggest that current maternal nutritional status, as measured by weight gain during pregnancy, may influence preterm delivery and gestation duration. The inconsistent results obtained with prepregnancy body mass may reflect a size bias inherent in the obstetric estimate of gestation, rather than effects of prepregnant maternal nutritional status on gestation.