The effect of excessive maternal weight gain on insulin levels during pregnancy (15 cases) was compared with the insulin levels in a control group (20 normal pregnancies) by radio immunologic determination of insulin in the amniotic fluid and in maternal and fetal blood. The mean values of insulin in the control group were as follows: mother 24.9 ± 0.51, fetus 10.86 ± 0.48 and amniotic fluid 6.08 ± 0.42 µU/ml, and in patients with excessive weight gain during pregnancy, maternal insulin mean values were 39.45 ± 1.86, fetal 20.99 ± 3.39 and amniotic fluid 12.25 ± 1.15 µU/ml. Insulin concentrations in maternal blood (P < 0.001), fetal blood (P < 0.001) and amniotic fluid (P < 0.001) were higher in excessive weight gain pregnancies as compared to the control group.