The criteria for the diagnosis of GDM are based on the original work of O’Sullivan and Mahan and modified by Carpenter and Coustan (see section on Gestational Diabetes). Risk assessment for GDM is undertaken at the first prenatal visit. Women with risk factors, including marked obesity, personal history of GDM in prior pregnancy, glycosuria, or strong family history, should have a glucose tolerance test (GTT) as soon as feasible. If results of testing do not demonstrate diabetes, they should be retested between 24 and 28 weeks’ gestation. A fasting plasma glucose of greater than 126 mg/dL or a casual level of greater than 200 mg/dL meets the criteria for diabetes if confirmed on a subsequent day.

Evaluation of low-risk women during pregnancy takes place between 24 and 28 weeks’ gestation and typically follows a two-step approach. An initial screening is a blood glucose concentration 1 hour after the patient takes a 50-gram oral glucose load. A value of greater than 140 mg/dL identifies approximately 80% of women with GDM. If a screening value is greater than 190 mg/dL, a fasting blood glucose should be checked on a subsequent day. A subsequent fasting value of ≥ 95 mg/dL would provide two abnormal values, as described below.

If the result of the 1-hour screening test falls between 141 and 190 mg/dL, a diagnostic 3-hour oral glucose test is performed. The 3-hour 100-gram oral glucose test is done after an overnight fast for at least 8 hours. Abnormal values are:

• Fasting ≥ 95 mg/dL
• 1-hour ≥ 180 mg/dL
• 2-hour ≥ 155 mg/dL
• 3-hour ≥ 140 mg/dL

At least two out of four values must be abnormal to diagnose gestational diabetes.