Background: Impaired healing of the uterine scar after cesarean has been associated with adverse gynecological and obstetric outcomes. Although a large number of studies have been conducted on the events leading to this, information obtained from prospective randomized studies examining the role of suture material in the formation of cesarean scar defect (CSD) is lacking. Objective: To evaluate the effects of synthetic suture materials on CSD formation. Study design: We performed a two-arm 1:1 randomized study in women with singleton pregnancies undergoing elective primary cesarean delivery after the 38th week of gestation. Uterine scar closure was performed using synthetic absorbable monofilament and multifilament sutures. The primary outcome was residual myometrial thickness (RMT) in the area of the scar, measured by transvaginal ultrasound 6-9 months after birth. Secondary outcomes included differences in mean operative time, mean estimated blood loss at the time of surgery, and the rates of postoperative gynecological sequelae. Results: Complete follow-up was obtained from 94 (88%) of 107 participants. RMT was thicker in the monofilament compared to the multifilament suture group (5.5 +/- 2.24 vs. 4.18 +/- 1.76, p = 0.01). Hemoglobin delta was higher in the monofilament suture group (1.59 +/- 0.96 vs. 1.25 +/- 0.60, p = 0.04). There was no statistically significant difference between the monofilament suture and multifilament suture groups in terms of gynecological sequelae. Conclusion: Closure of the uterine scar with monofilament suture has a positive effect on scar healing and increases RMT thickness.