Obesity prevalence in the United States has reached an alarming level. Consequently, more young women are entering pregnancy with body mass indices of at least 30 kg/m(2). While higher maternal weight entering pregnancy is related to several adverse pregnancy outcomes, some of the strongest and most compelling data to date have linked prepregnancy obesity to gestational diabetes mellitus (GDM). The mechanisms by which excess maternal weight influences metabolic dysfunction in pregnancy are similar to those in obese nonpregnant women; adipocytes are metabolically active and release a number of hormones implicated in insulin resistance. Heavier mothers are also more likely to have higher glucose levels that do not exceed the cutoff for GDM, but nevertheless predict poor perinatal outcomes. Longer-term complications of GDM include increased risk of maternal type 2 diabetes and offspring obesity. Promising intervention studies to decrease the intergenerational cycle of obesity and diabetes are currently underway.