The evolution of gestational diabetes mellitus (GDM) diagnosis starts at the 1960s. Since then it has been the focus of ongoing debate, regarding questions such as - when, how and who to screen? What is the most appropriate diagnostic procedure? What cutoffs to diagnose GDM? By far, the hyperglycemia and adverse pregnancy outcome (HAPO) study provided the best evidence base to support answers to some of these questions - yet, some remain unsolved and others are still a matter of debate. Translating clinical evidence to practice guidelines is not a pure matter of science, but other considerations come into play, especially when the associations between the diagnostic variables to the selected outcomes are linear. Here, we shall provide an overview of the pathway which has led to HAPO and the path taken since its publication, striding toward a consensus for GDM diagnosis.