Intrauterine growth retardation and low birthweight--factors that strongly influence the physical and mental development of a child--are in turn affected by the nutritional status of the mother during pregnancy and, to a certain extent, by her pregestational nutritional status. Pregnant adolescents constitute a high-risk group for nutritional problems because their own bodies are still growing. In order to examine the correlation between several variables related to body composition and nutritional status in a group of pregnant adolescents and certain indicators of neonatal development, a prospective longitudinal study was carried out in Valdivia, Chile, from September 1988 to May 1992. The study cohort was made up of 184 pairs consisting of mothers under 17 years of age who had attended a prenatal monitoring program and their newborns. The following groups of variables were tested for correlation: indicators of maternal body composition before pregnancy (pregestational weight recorded by the mother, height measured during the first visit to the program, and body mass index [pregestational weight/(height upon entering the program)]; indicators of maternal body composition during pregnancy (weight and body mass index upon entering the program and before giving birth, weekly weight gain, and total weight gain); and indicators of neonatal development (weight and length at birth, gestational age, and cranial perimeter). The weight of the mother before giving birth was statistically significantly correlated with the gestational age, length, weight, and cranial perimeter of the newborn. The body mass index prior to giving birth was weakly correlated with the weight and length of the newborn, and a significant direct correlation was also observed between the weight of the pregnant adolescent upon entering the program and the weight of her child at birth. No correlation was found between the indicators of fetal development and those of maternal pregestational body composition or nutritional status. These results show that interventions conducive to a good increase in maternal weight during pregnancy will help prevent a bad neonatal prognosis.