IN BRIEF Traditional methods of screening for and diagnosing gestational diabetes mellitus (GDM) have been challenged, leading to the development of new screening guidelines by the International Association of Diabetes and Pregnancy Study Groups (IADPSG). This study is a retrospective comparison of pregnancy outcomes based on the Carpenter and Coustan (CC) and IADPSG screening guidelines. It demonstrates that adoption of IADPSG guidelines increased the rate of GDM diagnosis and resulted in more women with diet-controlled GDM. However, a decrease in adverse pregnancy outcomes was not noted. Interestingly, women diagnosed by IADPSG criteria who had diet-controlled GDM had less maternal weight gain than those screened with the CC criteria who did not have GDM but did have an elevated 1-hour oral glucose tolerance test result.