Background <p>A uterine niche (cesarean section scar defect), defined as an indentation ≥ 2&#xa0;mm in the myometrium at the site of a previous cesarean scar, reflects impaired myometrial healing after cesarean delivery. Its prevalence increases with the number of cesarean deliveries and is associated with abnormal uterine bleeding, pelvic pain, secondary infertility, cesarean scar ectopic pregnancy, placenta accreta spectrum disorders, and uterine rupture in subsequent pregnancies. While secondary surgical correction may alleviate symptoms, preventive strategies implemented at the time of repeat cesarean delivery remain insufficiently studied. This trial evaluated whether resection of the previous cesarean scar during repeat cesarean delivery reduces uterine niche formation and related morbidity without increasing operative risk.</p> Methods <p>This single-center, prospective, parallel-group, randomized controlled superiority trial was conducted at a tertiary university hospital between February and November 2025. A total of 170 multigravida women aged 18–45 years with ≥ 1 prior cesarean delivery undergoing elective or urgent repeat cesarean section were randomized (1:1) to either repeat cesarean delivery with resection of the previous cesarean scar (scar-resection group, <i>n</i> = 85) or standard repeat cesarean section without scar resection (control group, <i>n</i> = 85). Outcome assessors were blinded to group allocation. The primary outcome was the incidence of uterine niche (≥ 2&#xa0;mm) at 6 months postpartum, assessed using saline-infusion sonohysterography. Secondary outcomes included operative time, additional hemostatic sutures, estimated blood loss, niche dimensions, residual myometrial thickness, and cyclic menstrual bleeding abnormalities at 6 months postpartum. Analyses were conducted on a modified intention-to-treat basis excluding participants lost before outcome assessment.</p> Results <p>A total of 160 participants (80 per group) completed the 6-month assessment. Operative outcomes, including total operative time, number of additional hemostatic sutures, and estimated intraoperative blood loss were comparable between groups (all <i>P</i> &gt; 0.05). At 6 months postpartum, the incidence of uterine niche was significantly lower in the scar-resection group than in the control group (45.0% vs. 78.8%; relative risk 0.57, 95% CI 0.44–0.75; <i>P</i> &lt; 0.001), corresponding to an absolute risk reduction of 33.8% and a number needed to treat of 3. These results remained significant after adjustment for number of previous cesarean deliveries. Among women who developed a niche, scar resection was associated with significantly smaller niche depth, length, and width, greater residual myometrial thickness, and significantly fewer postmenstrual or intermenstrual spotting days and total bleeding days per cycle (all <i>P</i> &lt; 0.0001).</p> Conclusions <p>Resection of the previous cesarean scar during repeat cesarean delivery significantly reduces the incidence and severity of uterine niche formation, improves residual myometrial thickness, and decreases niche-related menstrual morbidity, without increasing operative time or blood loss. This standardized surgical modification represents a feasible preventive strategy to reduce cesarean scar–related morbidity.</p> Trial registration <p>ClinicalTrials.gov, NCT07228858. Registered 20 December 2025 (retrospectively registered).</p>