While the contribution of total maternal gain to birthweight is well described, less is known about whether there are specific effects of early weight gain. Early weight gain adequacy may be a particular problem among teenage gravidas who are more likely to have low prepregnant weight and in developing countries where chronic undernutrition is endemic. We studied the effects of early weight gain adequacy on infant birthweight in a geographical cohort of 1,790 adolescents from Camden County, New Jersey. Weight gain was calculated for prepregnant weight to 24 weeks' gestation and 24 weeks to delivery. Inadequate early weight gain was defined as gains less than 4.3 kg and inadequate late gains as averaging less than 400 @week from 24 weeks' gestation to delivery. In regressions predicting birthweight outcome, we found significant independent effects of timing of weight gain inadequacy. Early inadequate gains were associated with a -186.6 ? 31.6 gm decrement in birthweight that was not diminished even when later weight gains were compensatory. Late inadequate (-154.4 i_ 29.3 gm) and inadequate gains both early and later (-298.6 2 49.1 gm) were also associated with birthweight decrements. However, overweight (body mass index >24.5) appeared to buffer some of the effects of inadequate weight gains, reducing the decrement t o about -100 gm. Protocols that seek to improve infant outcomes should focus on early weight gain during pregnancy. Affecting weight gain only late in pregnancy may not be able to substantially reduce the risk of fetal growth retardation.