Association of pre‐pregnancy body mass index and gestational weight gain on postpartum pelvic floor muscle morphology and function in Chinese primiparous women: A retrospective cohort study

Abstract Objective Our study aimed to investigate the association between maternal pre‐pregnancy body mass index (BMI), gestational weight gain (GWG), and impaired pelvic floor muscle (PFM) morphology and function during the early postpartum period. Methods This retrospective cohort study was conducted at Shanghai First Maternity and Infant Hospital from December 2020 to December 2022. A total of 1118 primiparous women with singleton pregnancies who underwent vaginal deliveries and participated in postpartum PFM assessments were included. Maternal pre‐pregnancy BMI and GWG were considered as exposures. PFM morphology and function impairment were the primary outcomes. PFM morphology impairment, defined as levator ani muscle avulsion, was assessed using transperineal ultrasound. PFM function impairment, manifested as diminished PFM fiber strength, was assessed through vaginal manometry. Multivariable logistic regression analysis was employed to calculate adjusted odds ratios (aOR) with 95% confidence intervals (CI). Restricted cubic spline models were used to validate and visualize the relationship. Results Women with lower pre‐pregnancy BMI were at an increased risk of levator ani muscle avulsion (aOR = 1.73, 95% CI: 1.10–2.70, P = 0.017), particularly when combined with excessive GWG during pregnancy (aOR = 3.20, 95% CI: 1.15–8.97, P = 0.027). Lower pre‐pregnancy BMI was also identified as an independent predictor of PFM weakness (aOR = 1.53, 95% CI: 1.08–2.16, P = 0.017 for type I fiber injuries). Notably, regardless of the avulsion status, both underweight and overweight/obese women faced an elevated risk of reduced PFM strength (aOR = 1.74, 95% CI: 1.17–2.59, P = 0.006 for underweight women with type I fiber injuries; aOR = 1.67, 95% CI: 1.06–2.64, P = 0.027; and aOR = 1.73, 95% CI: 1.09–2.76, P = 0.021 for overweight/obese women with type I and type II fibers injuries, respectively). Conclusions Both lower and higher pre‐pregnancy BMI, as well as excessive GWG, were strongly associated with PFM impairments. These findings highlighted the critical importance of comprehensive weight management throughout pregnancy to effectively promote women's pelvic health.