The aim of this study is to characterize changes in body composition during pregnancy in women with obesity.Fifty-four healthy women with obesity (class 1, 30-34.9 kg/m : n = 25; class 2, 35-39.9 kg/m : n = 21; class 3, ≥ 40.0 kg/m : n = 8) expecting a singleton pregnancy were studied. Body composition was measured in early pregnancy (13-16 weeks), midpregnancy (24-27 weeks), and late pregnancy (35-37 weeks) using air displacement plethysmography, stable isotopes, and skinfold thickness measurements. Fasting glucose, insulin, and leptin were measured.The gain in fat-free mass was lower in the second trimester compared with the third (2.7 ± 0.2 to 5.3 ± 0.2 kg; P < 0.001), whereas fat mass accumulation declined over time (0.6 ± 0.3 to -0.7 ± 0.4 kg; P = 0.005). Women with class 1 and 2 obesity gained 1.1 ± 0.7 kg of fat mass during pregnancy, while women with class 3 obesity lost 4.1 ± 0.6 kg (both P < 0.001). The difference in fat accumulation between obesity classes was observed only in the second trimester (P = 0.02). Gestational weight gain was associated positively with changes in plasma concentrations of insulin, leptin, and insulin resistance (all P < 0.01).Gestational weight gain in pregnancy differs by obesity class and trimester. Women with class 3 obesity gain less body weight and fat mass. Fat mass gain is most likely preventable in the second trimester.