A Single‐Center Retrospective Cohort Study of Risk Factors for Obstetric Anal Sphincter Injury at a Tertiary Hospital in Japan

AIM: To identify maternal and intrapartum risk factors associated with obstetric anal sphincter injury (OASIS) among Asian women who underwent singleton term vaginal delivery at a tertiary perinatal center in Japan. METHODS: We conducted a retrospective cohort study including 2334 Asian women who underwent singleton term vaginal delivery at Kitasato University Hospital between January 2020 and December 2024. Maternal characteristics and intrapartum variables were extracted from the institutional perinatal database and electronic medical records. Univariate and multivariable logistic regression analyses were performed to identify independent risk factors for OASIS. RESULTS: Among 2334 singleton term vaginal deliveries, OASIS occurred in 101 women (4.3%). Multivariable logistic regression analysis demonstrated that nulliparity (adjusted odds ratio [aOR] 4.86, 95% confidence interval [CI] 2.47-9.58), instrumental delivery (aOR 4.02, 95% CI 2.19-7.37), and abnormal cardiotocography (CTG) findings (aOR 2.17, 95% CI 1.39-3.37) were independently associated with OASIS. CONCLUSION: Abnormal CTG findings may represent a potentially novel risk factor for OASIS among Asian women, while nulliparity and instrumental delivery were confirmed as established risk factors. Increased attention to perineal protection may be warranted in women with these risk factors during vaginal delivery.