Objective: This study prospectively evaluated the longitudinal changes in insulin sensitivity, insulin response, and endogenous (primarily hepatic) glucose production and suppression during insulin infusion in women with normal glucose tolerance (control) and gestational diabetes mellitus before and during a planned pregnancy. Study Design: Eight control subjects and 7 subjects in whom gestational diabetes mellitus developed were evaluated with an oral glucose tolerance test, an intravenous glucose tolerance test, and hyperinsulinemic-euglycemic clamp with infusion of [6,6 ²H₂ ]glucose before conception and at 12 to 14 and 34 to 36 weeks’ gestation. Insulin response was estimated as the area under the curve during the intravenous glucose tolerance test. Basal endogenous glucose production was estimated from isotope tracer dilution during steady state with [6,6 ²H₂ ]glucose and suppression during insulin infusion. Insulin sensitivity to glucose was defined as the glucose infusion rate required to maintain euglycemia during steady-state insulin infusion. Body composition was estimated with hydrodensitometry. Data were analyzed with 2-way analysis of variance with repeated measures for 2 groups. Results: There were increases in first-phase (P = .006) and second-phase (P = .0001) insulin responses in both groups with advancing gestation, but the increase in second-phase response was significantly greater (P = .02) in the gestational diabetes mellitus group than in the control group. Basal glucose production increased significantly (P = .0001) with advancing gestation, and there was resistance to suppression during insulin infusion in both groups (P = .0001). There was less suppression of endogenous glucose production however, in the gestational diabetes mellitus group than in the control group (P = .01). Insulin sensitivity decreased with advancing gestation in both groups (P = .0001), and there was lower insulin sensitivity in the gestational diabetes mellitus group than in the control group (P = .04). Significant decreases in insulin sensitivity with time (P = .0001) and between groups (P = .03) remained when the data were adjusted for differences in insulin concentration or residual hepatic glucose production. Conclusion: Obese women in whom gestational diabetes mellitus develops have a significant increase in insulin response but decreases in insulin sensitivity and suppression of hepatic glucose production during insulin infusion with advancing gestation with respect to a matched control group. These metabolic abnormalities in glucose metabolism are the hallmarks of type 2 diabetes, for which these women are at increased risk in later life. (Am J Obstet Gynecol 1999;180:903-16.)