Gestational diabetes mellitus (GDM) is carbohydrate intolerance that begins or is first recognized during pregnancy. Although the prevalence is usually reported as 2 to 5 percent of pregnant women, it can be as high as 14 percent in high-risk groups. These women are at risk of developing diabetes and related conditions later in life and face a range of complications during pregnancy, including hypertension, preeclampsia, and cesarean delivery. Macrosomia is more common in infants exposed to GDM, as are the risks of operative delivery, shoulder dystocia, birth trauma, and obesity during childhood.

Although the U.S. Preventive Services Task Force concluded that there was insufficient evidence to recommend universal screening for GDM, more than 90 percent of obstetric physicians report screening all patients. Screening based on risk factors such as obesity, family or personal history of diabetes, or previous adverse pregnancy outcome misses approximately one half of mothers with GDM. Some experts advocate random blood glucose screening for GDM using 95 to 105 mg per dL (5.3 to 5.8 mmol per L) fasting glucose level, 180 to 190 mg per dL (10.0 to 10.5 mmol per L) after one hour, or 155 to 165 mg per dL (8.6 to 9.2 mmol per L) after two hours as diagnostic criteria. However, no standard criteria for random blood glucose levels have been agreed on, and the test lacks sufficient sensitivity for screening. The most commonly used test is the 50-g one-hour glucose challenge. The accepted optimal cut-off points vary, but at the recommended level of 130 mg per dL (7.2 mmol per L), the screening test has an estimated sensitivity of 79 percent and a specificity of 87 percent. The diagnostic test specific to pregnancy and with the most supporting data is the 100-g three-hour oral glucose tolerance test

Women with GDM are often treated initially with diets designed to achieve normal glycemic levels and avoid ketoacidosis. However, the optimal diet has not been determined, and calorie restriction may increase the chance of ketosis. Several trials have demonstrated reduced fetal macrosomia if the mother is treated with insulin. Although insulin treatment is common in GDM, only 9 to 40 percent of treated mothers benefit. Treatment aims to achieve glucose levels of 130 mg per dL one hour postprandially. Oral hypoglycemic agents, with the exception of glyburide, are contraindicated in pregnancy. In one study, glyburide provided outcomes comparable to those achieved with insulin in patients with GDM who had failed to achieve adequate glycemic control with diet alone.

When glycemic control is satisfactory and no complications occur, mothers with GDM routinely do not require early or operative delivery. Nevertheless, the high incidence of macrosomia and other complications often results in cesarean or other operative delivery. Ultrasonography performs poorly in the prediction of birth weight in infants with macrosomia.