Background: Gestational Diabetes Mellitus (GDM) is a common metabolic disorder during pregnancy with significant maternal and fetal complications. The Oral Glucose Tolerance Test (OGTT) is the gold standard for diagnosis but has limitations such as fasting requirements and patient discomfort. HbA1c, a measure of long-term glycemic control, has been proposed as an early screening tool. We evaluated the effectiveness of first-trimester HbA1c in screening for GDM and its correlation with pregnancy outcomes. Methods: A prospective cohort study was conducted on pregnant women attending antenatal clinics at 8–12 weeks of gestation. Participants underwent OGTT and HbA1c testing. GDM was diagnosed using DIPSI criteria. Statistical analysis included ROC curve evaluation to determine optimal HbA1c cutoff values. Results: An HbA1c threshold of 5.3% demonstrated high negative predictive value (96.2%) and specificity (71.43%). However, no significant differences were found in maternal age, BMI, prenatal complications, perinatal outcomes, or delivery methods between those with HbA1c <5.3% and ≥5.3%. Conclusion: While HbA1c alone is not sufficient to replace OGTT, it can aid in early risk stratification and reduce the need for extensive OGTT screening. Population-specific cutoff values should be established for accurate diagnosis. Further large-scale studies are necessary to determine its role in predicting adverse pregnancy outcomes.