Normal pregnancy is associated with reduced insulin sensitivity. Gestational diabetes (GDM) occurs when insulin secretion is insufficient to compensate for the insulin resistance of pregnancy. Gestational diabetes mellitus (GDM) is defined as any degree of glucose intolerance, first time detected in pregnancy. Gestational diabetes mellitus is a risk factor for perinatal complications. Early diagnosis of the disease may reduce fetal exposure to maternal hyperglycemia and decrease the risk of complications. Even very mild alterations in glucose tolerance can result in excessive or disharmonious fetal growth. The diagnosis of GDM should initiate monitoring of glucose tolerance to minimize the risk of developing overt diabetes. Any degree of abnormal glucose homeostasis in pregnancy independently predicts an increased risk of glucose intolerance postpartum. Gestational diabetes possibly be present in women with maturity onset diabetes of the young (MODY) or in early phase type LADA diabetes.