Infant macrosomia is a classic feature of a gestational diabetes mellitus (GDM) pregnancy and is associated with increased risk of adult obesity and type II diabetes mellitus, however mechanisms linking GDM and later disease remain poorly understood. The heterozygous leptin receptor-deficient (Lepr(db/+)) mouse develops spontaneous GDM and the fetuses display characteristics similar to infants of GDM mothers. We examined the effects of GDM on maternal insulin resistance, fetal growth, and postnatal development of hepatic insulin resistance. Fetal body weight on d 18 of gestation was 6.5% greater (p < 0.05) in pups from ad libitum-fed db/+ mothers compared with wild-type (WT) controls. Pair-feeding db/+ mothers to the intake of WT mothers normalized fetal weight despite less than normal maternal insulin sensitivity. More stringent caloric restriction reduced insulin and glucose levels below WT controls and resulted in fetal intrauterine growth restriction. The level of hepatic insulin receptor protein was decreased by 28% to 31% in both intrauterine growth restriction and fetuses from ad libitum-fed GDM mothers compared with offspring from WT mothers. In 24-wk-old adult offspring from GDM mothers, body weight was similar to WT offspring, however, the females from GDM mothers were fatter and hyperinsulinemic compared with offspring from WT mothers. Insulin-stimulated phosphorylation of Akt, a key intermediate in insulin signaling, was severely decreased in the livers of adult GDM offspring. Hepatic glucose-6-phosphatase activity was also inappropriately increased in the adult offspring from GDM mothers. These results suggest that spontaneous GDM in the pregnant Lepr(db/+) mouse is triggered by overfeeding, and this effect results in obesity and insulin resistance in the livers of the adult offspring. The specific decrease in Akt phosphorylation in livers of adult offspring suggests that this may be a mechanism for reduced insulin-dependent physiologic events, such as suppression of hepatic glucose production, a defect associated with susceptibility to type II diabetes mellitus.