Background: Cesarean scar defects (CSD) are associated with significant maternal morbidity, yet the impact of different uterine closure techniques on CSD formation remains underexplored. This study evaluates the effect of double uterine closure with and without the inclusion of the upper peritoneal flap on CSD size and residual myometrial thickness (RMT). This study aims to compare the residual myometrial thickness and the size of the cesarean scar defect following double-layer uterine closure with and without the inclusion of the upper peritoneal flap. Methods: This double-blind, prospective observational study included 70 women undergoing primary cesarean section at term. Participants were randomly assigned to two groups: Group A (n=35) received double-layer uterine closure without peritoneal inclusion, and Group B (n=35) received closure with peritoneal inclusion. Transvaginal ultrasonography was used to measure CSD size and RMT at 1, 3, and 6 months postoperatively. Results: At 6 months, Group B demonstrated significantly smaller CSD size (2.95±0.58 mm vs. 3.51±0.76 mm, p=0.04) and greater RMT (5.65±0.7 mm vs. 4.95±0.8 mm, p=0.02) compared to Group A. ROC curve analysis revealed a CSD size cut-off of