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To evaluate if any single plasma glucose level from the four values of the normal 100-g oral glucose tolerance test (OGTT) in early pregnancy (≤20 weeks of gestation) could predict gestational diabetes mellitus (GDM) diagnosed from a second OGTT in late pregnancy (28–32 weeks).Glucose levels of pregnant women at high-risk for GDM, who had had a normal early OGTT, and who underwent the second test in late pregnancy, were studied. Each of the four plasma glucose values of the early OGTT was determined for sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). The receiver operating characteristic curves of these four OGTT values were then constructed to find the optimal value to predict late-onset GDM.Of 193 pregnant women who had had a normal early OGTT, 154 also had a normal OGTT in late pregnancy while 39 had an abnormal test and were diagnosed with GDM. Among the four glucose values of the early OGTT, the 1-h value yielded the best diagnostic performance to predict late-onset GDM. The sensitivity, specificity, PPV, NPV, and area under the curve achieved from its optimal cutoff level of ≥155 mg/dL (8.6 mmol/L) were 89.7%, 64.3%, 38.9%, 96.1%, and 0.77, respectively.A 1-h glucose value ≥155 mg/dL at the early OGTT yielded the best diagnostic performance. However, the low specificity and PPV rendered it suboptimal to predict late-onset GDM. Nevertheless, a considerable number of high-risk women could avoid the second OGTT in late pregnancy due to its high sensitivity and NPV.