Gestational diabetes mellitus (GDM), defined as carbohydrate intolerance that begins or is first recognized during pregnancy, is associated with increased maternal, fetal, and neonatal risks. The prevalence of GDM in the United States is increasing, probably because of increasing rates of overweight and obesity. A universal recommendation for the ideal approach for screening and diagnosis of GDM remains elusive. At this time, the Committee on Obstetric Practice continues to recommend a two-step approach to screening and diagnosis. All pregnant women should be screened for GDM, whether by patient history, clinical risk factors, or a 50-g, 1-hour glucose challenge test at 24–28 weeks of gestation. The diagnosis of GDM can be made based on the result of the 100-g, 3-hour oral glucose tolerance test, for which there is evidence that treatment improves outcome.