Clinical data demonstrated a close relationship between the conceptional glycated haemoglobin and the occurrence of spontaneous early abortions and fetal malformations. Blood glucose level during pregnancy was correlated with the risk of fetal macrosomia with significant links between birth weight, fetal hyperinsulinemia and mean maternal blood glucose levels. Tight blood glucose control from conception to term was clearly shown to be able to lower the risk of fetal abnormalities and perinatal mortality to that of the offspring of a non-diabetic mother. But this imply mainly to plan the pregnancy and to maintain a tight normoglycemia from the conception, and to establish a close partnership between diabetologist and obstetrician. The diagnosis and the management of gestational diabetes mellitus (GDM) are still subject to a large debate, and particularly screening and diagnosis criteria. However, diagnosis and management of GDM are clearly mandatory, particularly to identify the undiagnosed type 2 diabetic women. As well as for a diabetic pregnancy, the therapeutic management of GDM must be instituted early and must be intensive in order to reduce the level of fetal macrosomia to the one of the non-diabetic population.