To assess the longitudinal changes in insulin release and insulin sensitivity in nonobese normal women during gestation, six women were evaluated with oral glucose tolerance testing, body composition analysis, intravenous glucose tolerance tests, and the hyperinsulinemic-euglycemic clamp before conception, at 12 to 14 weeks, and at 34 to 36 weeks' gestation. There was a significant increase in the insulin/glucose ratio (p = 0.028) during the oral glucose tolerance test during gestation. There was also a significant 3.0- to 3.5-fold increase throughout gestation in first-phase (p = 0.001) and second-phase (p = 0.0001) insulin release during the intravenous glucose tolerance test. Peripheral insulin sensitivity was estimated as the glucose infusion rate (in milligrams per kilogram fat-free mass per minute) during the hyperinsulinemic-euglycemic clamp. There was a significant (p = 0.0003) 56% decrease in insulin sensitivity through 36 weeks' gestation. These results are the first to prospectively evaluate the longitudinal changes in maternal carbohydrate metabolism from the time before conception through late gestation with newer methods such as the hyperinsulinemic-euglycemic clamp.