Gestational diabetes mellitus is characterised by glucose intolerance of variable severity that begins or is first diagnosed during pregnancy and usually resolves not long after delivery. Unknown type 2 diabetes mellitus must be diagnosed in women with risk factors, ideally before pregnancy, otherwise at first prenatal visit. Universal screening of gestational diabetes mellitus is performed between 24 and 28 weeks of gestation with a 75 g 2-hour oral glucose tolerance test. Thresholds are 0.95 g/ and/or 1.55 g/l, respectively and could be mildly modified soon. The evidence clearly supports the treatment of gestational diabetes mellitus and the modifications of obstetrical management in these women. According to the increased risk of the subsequent development of type 2 diabetes, fasting glycemia should be performed at 6-12 weeks postpartum and every three years. Obese and overweight women have increased risks of perinatal complications.