Association Between the Duration of the Second Stage of Labor and Postpartum Lateral Pelvic Tilt in Women with Singleton Cephalic Vaginal Delivery

Objective: To investigate the association between the duration of the second stage of labor and postpartum lateral pelvic tilt and its underlying mechanisms in women with singleton cephalic vaginal deliveries. Methods: In this retrospective cohort study, 4541 women with singleton cephalic vaginal deliveries were included. Participants were stratified by a second-stage duration cutoff of 1.5 hours, and 1:1 propensity score matching yielded 1406 matched pairs. Univariate and multivariate logistic regression were used to assess variable associations with lateral pelvic tilt. Three restricted multivariable logistic models evaluated the independent effect of second-stage duration, while restricted cubic splines tested for nonlinear relationships with postpartum lateral pelvic tilt. Subgroup analyses examined effect heterogeneity, and mediation models assessed the intermediary roles of delivery mode and oxytocin use. Results: Both univariate and multivariate logistic regression analyses, including restricted multivariable models, consistently identified prolonged second-stage duration (≥1.5 hours) as an independent risk factor for lateral pelvic tilt (adjusted Odds Ratio [aOR] 1.34, 95% Confidence Interval [CI] 1.02-1.76, P < 0.05). Restricted cubic spline analysis revealed no significant nonlinear relationship (P>0.05). Subgroup analyses demonstrated enhanced associations among women undergoing operative vaginal delivery or receiving oxytocin. Mediation analysis confirmed delivery mode and oxytocin administration as significant mediating pathways, accounting for 11.07% and 13.09% of the total effect, respectively. Conclusion: Prolonged second-stage labor constitutes an independent risk factor for postpartum lateral pelvic tilt, partially mediated through operative delivery and oxytocin administration. Early pelvic evaluation and targeted intervention are recommended for women experiencing extended second stage, particularly those requiring instrumental assistance or oxytocin augmentation.