To examine the utility of the 75 g oral glucose tolerance test (OGTT), conducted according to the criteria of the International Association of Diabetes and Pregnancy Study Groups (IADPSG), for the early diagnosis of gestational diabetes mellitus (GDM) and to propose new cut-off values. A total of 350 prospectively enrolled patients were admitted to Inonu University School of Medicine Obstetrics and Gynecology Outpatient Clinic between April 2012 and January 2015 for first-trimester screening. Gestational diabetes mellitus (GDM) during the first trimester of pregnancy (11-13 weeks) was diagnosed using the 75-g OGTT. In patients who tested negative, the OGTT was repeated at 24-28 weeks. GDM was diagnosed in 14.6% of the patients, of whom 80.3% were diagnosed during the first trimester. In these patients, there were no remarkable changes in fasting plasma glucose level when a fasting glucose cutoff of 92 mg/dl was used for the diagnosis of GDM. The sensitivity and specificity of the OGTT were 66.6% and 99.3%, respectively (area under the receiver operating characteristic curve [AUROC] 0.892, 95% CI 0.855-0.923, p < 0.001). The cutoff value for a positive 75-g OGTT result was reduced from 180 to 173 mg/dl for the 1-h post-glucose load (AUROC 0.908, 95% CI 0.873-0.936, p < 0.001) and from 153 to 129 mg/dl for the 2-h post-glucose load (AUROC 0.861, 95% CI 0.515-0.775, p < 0.001). The 75-g OGTT based on IADPSG criteria can be used to detect 80% of GDM cases as early as the first trimester. A modification of current cutoff values would improve the sensitivity of the test but lower its specificity.