OBJECTIVE To determine the fat deposited during pregnancy in women gaining according to recommendations of the Institute of Medicine and the relationship of weight gain to fat gain in women of different starting weights (classified by their body mass index). METHODS A cohort study of healthy, nonsmoking women, 18-36 years of age, identified during prenatal visits at three hospital clinics and one birthing center in New York City. From a pool of 432 eligible volunteers who signed a consent form, body composition measurements were performed on 200 women at weeks 14 (+/-2) and 37+ of pregnancy, and bone mineral mass was measured at 2-4 weeks postpartum. Body fat was estimated with a model that used total body water, weight, and density and bone mineral mass. RESULTS In women gaining as recommended by the Institute of Medicine, fat gains during pregnancy for women underweight, normal weight, overweight, or obese before pregnancy were 6.0 +/- 2.6 kg, 3.8 +/- 3.4 kg, 3.5 +/- 4.1 kg, and -0.6 +/- 4.6 kg, respectively. Higher weight gain increased fat gain. Body water gain was not different among the four prepregnancy weight groups. CONCLUSIONS Recommended weight gain should not cause obesity in any weight group. Underweight women will normalize their body composition if they gain as recommended, whereas obese women will have little or no change in body fat. A majority of women do not gain as recommended during pregnancy.