Gestational diabetes mellitus (GDM) is defined as carbohydrate intolerance with onset or first recognition during pregnancy. If insulin resistance and hyperglycemia are recognized before pregnancy, the term pregestational diabetes (PGD) is used. In the last years, much has been invested to uniformly define worldwide the diagnostic criteria and the management of gestational diabetes. Recently, the American Diabetes Association stated that a general screening in a low risk population should be implemented into routine prenatal care. However, there are still controversies on which diagnostic criteria to use. Similarly, screening of pregnant women at high risk for PGD is also a matter of debate. This is an important issue as the prevalence of this group is increasing worldwide. Of interest, the ADA as well as the international expert committee on diabetes published a consensus statement in which the use of HbA1c was supported for the diagnosis of diabetes rather than the measurement of fasting or post-prandial plasma glucose in a non-pregnant population. A value of ≥6.5% (≥48mmol/mol) was proposed as criterion for diagnosis. There is no doubt that the measurement of glycosylated haemoglobin may be of value in particular preconceptional in diabetes type 1. Difference exist in using HbA1c during pregnancy. At the pregnant state, the different red cell turnover and other physiologic changes in pregnancy may interfere with HbA1c. Indeed, mean HBA1c levels in healthy pregnant women are lower than in the non-pregnant women. Moreover, O'Connor et al. showed that first and second trimester HbA1c values are significantly different to a non-pregnant population, whereas no difference was found comparing HbA1c values from the third trimester. Therefore, it has been suggested, that only blood glucose criteria should be used to diagnose or exclude PGD in particular during the first trimester as the data on HBA1c in the first trimester is limited .