Association Between Mode of Delivery and Levator Ani Muscle Abnormalities on 4D Transperineal Ultrasound at 6 Weeks Postpartum

Purpose: It is well established that vaginal delivery is the main risk factor for the development of pelvic floor disorders, which is mainly mediated by structural alterations in the levator ani muscle. However, there are no robust data on the magnitude of cesarean section correlated with the morphological changes of the levator ani muscle based on ultrasound anatomical markers, which is an interesting topic to focus on in this article. Materials and Methods: This retrospective study included 1930 women who delivered at our center between February 2024 and March 2025. The two groups were divided according to the mode of delivery (vaginal delivery and cesarean section), and analysis included baseline characteristics, pregnancy parameters, delivery details, and ultrasonic findings in the levator ani muscle and levator hiatus 6 weeks postpartum. Results: In the vaginal delivery group, 45 cases (3.7%) have levator ani muscles avulsion, while 887 cases (72.5%) with enlarged levator hiatus, the corresponding figures of cesarean section group were 2 cases (0.3%) and 295 cases (41.8%), respectively. On univariate analysis, we found significant differences in the baseline characteristics, obstetrical details, levator ani muscle avulsion and hiatal enlargement when comparing participants with vaginal deliveries versus women who underwent cesarean sections. After adjustment for confounding factors, multivariate logistic regression analysis shows women who delivered vaginally had a 6.58-times and 4.31-times odds of levator ani muscle avulsion as well as hiatal enlargement compared to women who underwent cesarean sections, respectively (OR = 6.58, 95% CI = 1.55– 27.87, P = 0.011; OR = 4.31, 95% CI = 3.30– 5.61, P < 0.001, respectively). Based on the dimensional evaluation for pelvic floor structures using 4D ultrasound, no association was noted between the levator avulsion as well as levator hiatus over-distension and cesarean section regardless of surgical indications ( P = 0.162 and P = 0.313, respectively). Conclusions: As a reliable, easily accessible and cost-effective methods, 4D ultrasonography has now become standard in recent years for the assessment of pelvic floor trauma. Therefore, our findings which are based on 4D ultrasound imply that vaginal delivery is an independent risk factor for levator ani muscle injury and over distension of the genital hiatus 6 weeks after childbirth.