Establishing that prenatal life is a critical or sensitive period for the development of obesity may focus basic research and clinical prevention efforts on this period. This review summarizes evidence that the intrauterine environment influences the risk of later obesity and considers the mechanisms by which this may occur. The association between birth weight and adult weight suggests that there are enduring effects of the intrauterine environment on later obesity risk. We examine whether the maternal factors of diabetes, obesity, and pregnancy weight gain alter the intrauterine environment and thereby increase the risk of later obesity in the offspring. Of these maternal factors, evidence is strongest for the role of maternal diabetes. No single mechanism explains how these maternal factors could change the intrauterine environment to increase obesity risk. However, all potential mechanisms involve an altered transfer of metabolic substrates between mother and fetus, which may influence the developing structure or function of the organs involved in energy metabolism.