Many factors may contribute to the high rates of low birth weight observed in most studies of teen pregnancy. This study examined the contribution of low prepregnancy weight and low gestational weight gain, both of which are more common in teens than in adults. Records were obtained for 90 primiparous teens (less than 16 years old at conception) and 90 primiparous adult women (19-30 years old at conception) delivering at St. Luke's Hospital in New York City. Women were matched for year of delivery and clinic payment status. Mean birth weight was 264 g lower in the teen group than in the adult group. In both groups, birth weight increased with increased maternal prepregnancy weight, weight gain, and percent of standard weight for height at term. In adults, birth weight plateaued when maternal weight for height was 120% or more of standard. In teens, however, birth weight was 200-400 g lower than in the adults in all weight-for-height categories except at 140% or more of standard. Both the low birth weight rates and the portion of infants weighing less than 3000 g were much higher in the teen mothers. Thus, even if maternal weight at term is appropriate for height, teen mothers have smaller babies than adult women do.