Gestational diabetes: A diabetic condition that appears during pregnancy and usually goes away after the birth of the baby. Gestational diabetes is best controlled by dietary adjustment. Gestational diabetes can cause birth complications. One complication is macrosomia, in which the baby is considerably larger than normal due to large deposits of fat; such a baby can grow too large to be delivered through the vagina. Gestational diabetes also increases the risk of low blood sugar, low serum calcium and low serum magnesium in the baby immediately after delivery. The key to prevention is careful control of the mother's blood sugar levels. If the mother maintains normal blood sugar levels, it is less likely that the fetus will develop macrosomia, hypoglycemia, or other chemical abnormalities.