The Hyperglycemia and Adverse Pregnancy Outcome (HAPO) Study showed significant perinatal risks at levels of maternal hyperglycemia below values that are diagnostic for diabetes. A Consensus Panel of the International Association of Diabetes and Pregnancy Study Groups (IADPSG) reviewed HAPO Study results and other work that examined associations of maternal glycemia with perinatal and long-term outcomes in offspring and published recommendations for diagnosis and classification of hyperglycemia in pregnancy in 2010. Subsequently, some commentaries and debate challenged the IADPSG recommendations. In this review, we provide details regarding some points that were considered by the IADPSG Consensus Panel but not published and address the following issues: 1) what should be the frequency of gestational diabetes mellitus (GDM); 2) were appropriate outcomes and odds ratios used to define diagnostic thresholds for GDM; 3) to improve perinatal outcome, should the focus be on GDM, obesity, or both; 4) should results of randomized controlled trials of treatment of mild GDM influence recommendations for diagnostic thresholds; and, 5) other issues related to diagnosis of GDM. Other groups are independently considering strategies for the diagnosis of GDM. However, after careful consideration of these issues, we affirm our support for the recommendations of the IADPSG Consensus Panel.