To investigate the determinants of fetal maturation of infants born to adolescent mothers, we studied the obstetric population attended at the Maternity Hospital of Lima, Peru. From this population we selected for inclusion in this study a sample of 412 adolescent mothers ranging in age from 13 to 15 years. These subjects were selected because the anthropometric measurements of their parents were obtained at the time the adolescents were being attended for delivery. This study indicates that gynecological age per se does not affect newborn maturity and newborn size. However, as inferred from multiple regression analyses, low gynecological age, when associated with incomplete maternal growth, does result in reduction of birth weight. This finding supports the hypothesis that among young immature stillgrowing adolescents there is a competition for nutrients between the growth needs of the mother and those of the fetus. Since this competition does not occur among adolescents who have completed their growth, the present findings can not be applied to all teenagers