The effects on neonatal outcome of maternal age, sociodemographic status, and prenatal health and behavior were assessed in a representative national sample of youth (National Longitudinal Survey of Work Experience of Youth). Primiparous women were categorized into four age-at-birth groups: 13 to 15 year old, 16 to 18 year old, 19 to 21 year old, and 22 to 30 year old. Younger mothers were lighter, gained less weight during pregnancy, and sought prenatal care later in their pregnancy. Neonates of the youngest mothers on average had lower birth weights, and shorter gestation periods. There were significant effects of maternal age, race, education, and pregnancy weight gain on the probability of giving birth to a premature or low-birth-weight infant. Our results also implicated the time of first prenatal care in the occurrence of premature delivery. We believe that interventions should emphasize the prevention of pregnancy in young adolescents and amelioration of the adverse prenatal conditions associated with living in lower socioeconomic conditions.