Gestational diabetes mellitus (GDM) is defined as glucose intolerance with the onset or first detection during pregnancy. About 135,000 cases of GDM are diagnosed annually in the United States. Important risk factors include higher maternal age, family history of diabetes, and increased pregravid body mass index (BMI). The prevalence of GDM in low-risk populations ranges from 1.4% to 2.8%; in high-risk populations, prevalence ranges from 3.3% to 6.1%. Markedly elevated maternal glucose levels most often occur in women with pregestational diabetes. Pregnant women with pregestational diabetes are at higher risk for multiple complications affecting both the mother and the fetus than those women without diabetes. Current therapy improves outcomes for both mother and neonate. The additional risk for adverse health outcomes attributable to the milder degrees of maternal hyperglycemia associated with GDM and the magnitude of the benefit from treating that risk are less certain. No well-designed and conducted randomized controlled trial (RCT) of screening for GDM has been completed, and thus the evidence for screening is indirect. National groups disagree about whether to recommend screening for GDM. Despite no strong recommendations in favor of universal screening from the American College of Obstetricians and Gynecologists (ACOG), 94% of Fellows in office-based practices reported performing universal screening for GDM in 1996. Fellows performed this screening even though ACOG acknowledged the weakness in the evidence in both 1994 and 2000. With continued controversy around the advisability of GDM screening, the RTI-University of North Carolina Evidence-based Practice Center (RTI-UNC EPC) conducted a systematic evidence review to assist the U.S. Preventive Services Task Force (USPSTF) in reconsidering its 1996 review, which found insufficient evidence to recommend screening. We restricted this review to screening for GDM after 24 weeks' gestation, thus excluding both women with known pregestational diabetes and those who are discovered by symptoms earlier in pregnancy.