Diabetes mellitus is a chronic metabolic condition characterized by elevated blood glucose levels due to defects in insulin production, insulin action, or both. There are two main types of diabetes that can affect pregnant women: Type 1 diabetes mellitus (T1DM) and Type 2 diabetes mellitus (T2DM). T1DM is an autoimmune disorder in which the body’s immune system destroys insulin-producing cells in the pancreas, leading to an absolute insulin deficiency. T2DM, on the other hand, is characterized by insulin resistance, where the body becomes less effective at using insulin, often alongside a decline in insulin production. While T1DM is autoimmune, T2DM is influenced by lifestyle factors, such as diet and physical activity, and is often associated with obesity.
During pregnancy, hormonal changes cause increased insulin resistance to ensure that enough glucose is available to support fetal development. Normally, the maternal pancreas compensates by producing more insulin. However, in women with pre-existing diabetes or those who develop gestational diabetes, this mechanism fails, resulting in hyperglycemia. High blood glucose levels can cause complications for both the mother and the baby, requiring careful management.
Diabetes is becoming an increasing concern in pregnancy. While T1DM affects a smaller proportion of pregnant women, T2DM is becoming more common, driven by rising obesity rates and increasing maternal age. Studies show that 1-2% of pregnancies are complicated by pre-existing diabetes, with T2DM becoming more prevalent. Gestational diabetes mellitus (GDM), a form of diabetes that arises during pregnancy, affects roughly 8% of pregnancies in the United States and about 14% worldwide by International Diabetes Federation estimates. Though GDM usually resolves after delivery, women who experience it carry about a 10-fold higher lifetime risk of T2DM, so they are rescreened with a 75 g glucose tolerance test at 4 to 12 weeks postpartum and periodically thereafter.
GDM is screened for at 24 to 28 weeks of gestation. The two-step approach used in most of the United States starts with a non-fasting 50 g glucose challenge; a 1-hour value at or above the chosen threshold (130, 135, or 140 mg/dL) leads to a fasting 100 g, 3-hour oral glucose tolerance test, and two or more abnormal values are diagnostic. The one-step approach is a fasting 75 g, 2-hour test in which a single value at or above 92 mg/dL fasting, 180 mg/dL at 1 hour, or 153 mg/dL at 2 hours is diagnostic. Women with risk factors such as obesity, prior GDM, or a strong family history are tested at the first prenatal visit to pick up undiagnosed pre-existing diabetes. Treatment targets are the same for every form of diabetes in pregnancy: fasting glucose below 95 mg/dL and either a 1-hour postprandial value below 140 mg/dL or a 2-hour value below 120 mg/dL. Insulin is the first-line drug when diet and exercise fail, because it does not cross the placenta. Metformin and glyburide both cross the placenta: glyburide is associated with more macrosomia and neonatal hypoglycemia than insulin, and the long-term effects of fetal metformin exposure are still being studied, so both are second-line choices for women who decline or cannot use insulin.
