Aim: This study aimed to evaluate the effects of diabetes mellitus and cervical dilatation on cesarean section scar healing. Material and methods: This prospective study included pregnant women diagnosed with diabetes mellitus and healthy control pregnant women. The study group was divided into active labor and pre-active labor based on cervical dilatation, and the diabetic group was categorized into gestational diabetes and preexisting diabetes mellitus. Vaginal ultrasound was performed in the entire study group at six months postpartum, and the location of the cesarean scar was evaluated. Results: There were 170 participants in the study; 85 were diabetic, and the remaining 85 were healthy controls. Niche frequency in diabetes mellitus cases was not different from that in healthy controls (p = 0.420). The mean residual myometrial thickness, proximal residual myometrial thickness, and distal residual myometrial thickness were lower in the diabetic group (p = 0.001). Residual myometrial thickness and niche presence in the diabetic group with active labor was not statistically different from the diabetic group without active labor (p >0.05). Additionally, residual myometrial thickness was thinner in the gestational diabetes mellitus group than in the preexisting diabetes mellitus group (3.61 ± 1.78 mm vs. 4.76 ± 2.82 mm, p = 0.032). Conclusions: There was no significant difference in niche frequency between diabetic cases and healthy controls. When comparing the gestational diabetes mellitus group and the preexisting diabetes mellitus groups, there was no difference in the presence of niches. Niche presence was more common in diet-regulated diabetic patients compared with the insulin-regulated group. Cervical dilatation at the time of cesarean section did not affect niche presence in diabetic cases.