Risk factors for obstetric anal sphincter injury among nulliparous of advanced maternal age

Objective: Data regarding the risk factors for obstetrical anal sphincter injury (OASI) among nulliparous advanced maternal age (AMA) women are scarce. We aimed to evaluate the risk factors and the rate of OASI in this population. Methods: A retrospective case-control study of nulliparous women aged ≥35 who delivered vaginally between March 2011 and August 2021. The study cohort included nulliparous AMA women, aged 35 years or older, delivering vaginally a singleton, vertex neonate. We excluded cases with missing data. Maternal and intrapartum characteristics were compared between OASI and no-OASI groups. We matched groups to the earliest gestational age in which OASI has occurred. Results: A total of 3,635 women met inclusion criteria. Mean age of the participants was 38 years (range 35–56, interquartile range 36–39). The oldest age in which an OASI was diagnosed was 45. A total of 55 (1.5%) cases of OASI were diagnosed. Gestational age at delivery was higher in the OASI group compared to the no-OASI group (mean 39^6/7±1^2/7 vs. 39^1/7±2^3/7 weeks respectively, p=.039). The rate of induction of labor, oxytocin and epidural analgesia, prolonged second stage, mode of delivery, and rate of blood transfusion did not vary between groups. Estimated fetal weight was higher in the OASI group (3,324 ± 308 vs. 3,114 ± 566 g, p<.001). Sonographic abdominal circumference was higher in the OASI group (338 ± 13 vs. 328 ± 28 mm, p<.001). Mean birthweight was higher in the OASI group (3,286 ± 400 vs. 3,059 ± 541 g, p=.002) and the birthweight centile was higher as well (60 ± 26 vs. 52 ± 25, p=.024). In a multivariable regression analysis including birthweight, gestational age, operative vaginal delivery, epidural anesthesia, and hypertensive disorders–birthweight and hypertensive disorder were positively associated with an OASI odds ratio [OR 95% CI 1.6 (1.1–2.32) for every 500 g increase; 2.42 (1.006–5.82), respectively]. Epidural anesthesia was negatively associated with OASI [OR 95% CI 0.49) 0.27–0.87)]. Conclusions: Epidural anesthesia is the only modifiable risk factor and is protective against OASI among nulliparous AMA women. Birthweight and hypertensive disorders are also independent predictors of OASI in this population.