What is gestational diabetes? Gestational Diabetes is a form of diabetes that appears only during pregnancy. Between 6 and 9 percent of pregnant women develop gestational diabetes, according to the centers for disease control and Prevention (CDC). What causes gestational diabetes? Insulin is a hormone produced in the pancreas that regulates the body's metabolism of fats and carbs, and helps the body turn sugar into energy. Gestational diabetes occurs when hormones from the placenta block insulin, preventing the body from regulating the increased blood sugar of pregnancy effectively. This causes hyperglycemia (or high levels of sugar in the blood), which can damage the nerves, blood vessels and organs in your body. When does gestational diabetes usually begin? Gestational diabetes is usually evaluated between week 24 and week 28 of pregnancy.