Risk factors for obstetric anal sphincter injury: an exploratory Norwegian regional cohort study including amniotomy as a potential novel determinant.

BACKGROUND: Obstetric anal sphincter injury refers to the unintended tearing of the anal sphincter and/or rectal mucosa during vaginal delivery, a complication associated with significant morbidity. Well-known risk factors for obstetric anal sphincter injury include primiparity, high maternal age, instrumental delivery, oxytocin augmentation, and large fetal size. Recently, we identified amniotomy as a potential novel independent risk factor, regardless of parity. This study aims to further explore whether amniotomy increases the risk of obstetric anal sphincter injury including known determinants in singleton vaginal births after 34 weeks of gestation at Innlandet Hospital Trust, using a cohort study design.

METHODS: Retrospective cohort study using data from the Innlandet Perinatal Database, including all women who had singleton vaginal births after 34 weeks of gestation at Innlandet Hospital Trust from 2012 to 2022. We used multivariate logistic regression models to develop a risk-factor model for obstetric anal sphincter injury employing backward elimination with stratification for parity. We included established risk factors in the models, as well as the novel potential risk factor amniotomy.

RESULTS: The annual incidence of obstetric anal sphincter injury at the hospitals was stable around 2% in the study period and a total of 26,349 births were included. First, amniotomy was not found to be a risk factor for obstetric anal sphincter injury in this contemporary cohort. Second, instrumental delivery was identified as an independent modifiable risk factor for obstetric anal sphincter injury: vacuum extraction presenting relative risks (95% CI) of 2.72 (2.06-3.61) and 3.29 (2.09-5.17) in vaginally primiparous and parous women and forceps 5.97 (3.96-9.00) in vaginally primiparous women. Episiotomy and epidural analgesia were associated with reduced risks of obstetric anal sphincter injury, but only in vaginally primiparous women (relative risks: 0.53 (0.40-0.70) and 0.69 (0.55-0.86)).

CONCLUSIONS: Based on systematically collected data from our regional birth registry, this cohort study confirmed the association between established risk factors for obstetric anal sphincter injury, such as instrumental vaginal delivery. However, we could not identify amniotomy as a risk factor for obstetric anal sphincter injury, possibly due to changes in clinical practice and potential data limitations.