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Background: Gestational diabetes mellitus (GDM) is associated with adverse maternal and fetal outcomes, and the oral glucose tolerance test (OGTT) is the recommended test for its diagnosis. We evaluated the role of glycated hemoglobin (HbA1c) in screening and diagnosis of GDM and its correlation with adverse pregnancy outcomes.Subjects and Methods: In this prospective observational study, OGTT and HbA1c were performed in 500 antenatal women between 24 and 28 weeks of gestation; the pregnant women were followed up thereafter. Repeat OGTT and HbA1c were done in women with GDM at 6 weeks postpartum.Results: Among the 500 women, 45 were diagnosed with GDM, for an incidence of 9%. The mean HbA1c level in women with GDM was 6.2 ± 0.6%, whereas it was 5.4 ± 0.5% in those with normoglycemia. Women with GDM had a higher incidence of pregnancy-related complications compared with normoglycemic women. An HbA1c cutoff of 5.3% had a sensitivity of 95.6% and a specificity of 51.6% for the diagnosis of GDM and would have avoided OGTT in approximately half of antenatal women, while missing 5% of the women. However, those with an abnormal HbA1c will require a confirmatory OGTT, as 50% of normoglycemic women would be misclassified as having GDM by this approach. On repeat testing postpartum, two of 45 women (4.4%) had overt diabetes mellitus, whereas five (11.1%) had impaired glucose tolerance.Conclusions: Although HbA1c cannot replace OGTT in the diagnosis of GDM, it can be used as a screening test, avoiding OGTT in approximately 50% of women, if a cutoff of 5.3% is used. [ABSTRACT FROM AUTHOR]