Gestational diabetes mellitus (GDM) is defined as any degree of glucose intolerance with onset or first recognition during pregnancy. Women with GDM and their offspring have an increased risk of developing type 2 diabetes mellitus in the future. The global incidence of GDM is difficult to estimate, due to lack of uniform diagnostic criteria. Various diagnostic criteria have been proposed. The benefit of treating GDM has also been controversial. The clinical significance of treating maternal hyperglycemia was made evident in the Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) study. The HAPO study demonstrated that there is a continuous association of maternal glucose levels with adverse pregnancy outcomes and served as the basis for a new set of diagnostic criteria, proposed in 2010 by the International Association of Diabetes and Pregnancy Groups (IADPSG). According to these criteria the diagnosis of GDM is made if there is at least one abnormal value (≥92, 180 and 153 mg/dl for fasting, one-hour and two-hour plasma glucose concentration respectively), after a 75 g oral glucose tolerance test (OGTT).