Gestational diabetes mellitus (GDM) is defined as a carbohydrate intolerance, with onset or first recognition during pregnancy. The prevalence of GDM, and the occurrence of related complications, depends upon the definition of normal glucose values during gestation. The diagnostic criteria for GDM are controversial mainly because they lack correlation to outcome, be it maternal or perinatal. To date, there are no risk based guidelines to make the diagnosis of GDM and determine practice guidelines that are extrapolated from perinatal and maternal outcome and endpoints. To answer some of the above-mentioned controversies, the hyperglycemia and adverse pregnancy outcome study (HAPO) was planned and executed. This review outlines the results of the HAPO study, which demonstrates that fasting glucose levels and post 75 g OGTT are correlated to maternal, perinatal and neonatal outcomes. It is anticipated that the international recommendation for GDM diagnosis, will be shortly published.