Gestational diabetes mellitus (GDM) refers to an impaired glucose tolerance that occurs while a patient is pregnant. It is similar to type 2 diabetes mellitus in that the body makes insulin but is unable to utilize it as well as it used to. Incidence in the United States is estimated to be about 7% for any type of diabetes complicating pregnancy; 86% of those cases are patients with GDM. Incidence worldwide approaches 16.9%.

Risk factors include prior pregnancy with GDM, maternal obesity, race other than White, excessive weight gain in pregnancy, older than average maternal age, polycystic ovarian syndrome (PCOS), hypertension, preeclampsia, and preexisting glucose intolerance.

Testing in the United States is typically conducted between 24 and 28 weeks' gestation via a 50-g glucose load and a blood sugar check at the 1-hour mark. This typical definition means that the patient will not be diagnosed until after approximately 20 weeks' gestation. The reasoning is that if early blood glucose values are elevated, it may be related to underlying, previously unrecognized glucose intolerance or type 2 diabetes. Early screening (at < 20 weeks' gestation) for diabetes should be considered for any patients with a body mass index (BMI) over 30 and any of the following risk factors:

• Prior pregnancy with GDM
• PCOS
• Prior infant weighing 4000 g or more
• Hypertension
• Ethnicity with higher risk of GDM (African American, Latino, Native American, Asian American, Pacific Islander)
• History of cardiovascular disease and elevated hemoglobin A1c (greater than or equal to 5.7%)

GDM increases the risk of having a macrosomic baby as well as the risk of developing preeclampsia, neonatal hypoglycemia, hyperbilirubinemia, and fetal hyperinsulinemia and the incidence of cesarean delivery. Diagnosis of GDM increases the lifetime risk of the patient developing diabetes later in life. It also may increase the risk of the baby developing diabetes and obesity in their lifetime.

Classifications for GDM include:

• Diet controlled or A1 – blood sugars can be controlled with diet alone
• A2 – medication is needed to control blood sugars