Gestational diabetes mellitus (GDM) is defined as glucose intolerance first discovered in pregnancy. Pregestational diabetes mellitus refers to any type of diabetes diagnosed before pregnancy. Pregnant women with pregestational diabetes experience an increased risk of poor maternal, fetal, and neonatal outcomes. The extent to which GDM predicts adverse outcomes for mother, fetus, and neonate is less clear. Depending on the diagnostic criteria used and the population screened, the prevalence of GDM ranges from 1.1 to 25.5 percent of pregnancies in the United States. The incidence of GDM has increased over the past decades in parallel with the increase in rates of obesity and type 2 diabetes mellitus, and this trend is expected to continue. It is unclear how much the increase in obesity will affect the proportion of women diagnosed with overt diabetes during pregnancy versus transient pregnancy-induced glucose intolerance. GDM is usually diagnosed after 20 weeks' gestation when placental hormones that have the opposite effect of insulin on glucose metabolism increase substantially. Women with adequate insulin secreting capacity overcome this insulin resistance of pregnancy by secreting more endogenous insulin to maintain normal blood glucose. Women with less adequate pancreatic reserve are unable to produce sufficient insulin to overcome the increase in insulin resistance, and glucose intolerance results. Glucose abnormalities in women with GDM usually resolve postpartum, but commonly recur in subsequent pregnancies. Women with GDM have an increased risk of future development of overt diabetes. The cumulative incidence of diabetes after a diagnosis of GDM varies widely depending on maternal body mass index (BMI), ethnicity, and time since index pregnancy, and it may reach levels as high as 60 percent. When glucose abnormalities persist postpartum in a woman with GDM, her diabetes is recategorized as overt diabetes. When this occurs, the likelihood that this woman had pregestational (i.e., overt) diabetes increases, especially if the diagnosis of GDM occurred before 20 weeks' gestation and glucose levels were markedly elevated in pregnancy. Based on systematic reviews published in 2003 and 2008, the USPSTF concluded that there was insufficient evidence upon which to make a recommendation regarding routine screening of all pregnant women for GDM. The primary aims of this review were to (1) identify the test properties of screening and diagnostic tests for GDM, (2) evaluate the potential benefits and harms of screening at greater than or equal to 24 weeks and less than 24 weeks' gestation, (3) assess the effects of different screening and diagnostic thresholds on outcomes for mothers and their offspring, and (4) determine the effects of treatment in modifying outcomes for women diagnosed with GDM. The benefits and harms of treatments were considered in this review to determine the downstream effects of screening on health outcomes. The intent of this review was also to assess whether evidence gaps in the previous USPSTF reviews have been filled. Key questions include: Key Question 1: What are the sensitivities, specificities, reliabilities, and yields of current screening tests for GDM? (a) After 24 weeks' gestation? (b) During the first trimester and up to 24 weeks' gestation? Key Question 2: What is the direct evidence on the benefits and harms of screening women (before and after 24 weeks' gestation) for GDM to reduce maternal, fetal, and infant morbidity and mortality? Key Question 3: In the absence of treatment, how do health outcomes of mothers who meet various criteria for GDM and their offspring compare to those who do not meet the various criteria? Key Question 4: Does treatment modify the health outcomes of mothers who meet various criteria for GDM and their offspring? Key Question 5: What are the harms of treating GDM and do they vary by diagnostic approach?