Episiotomy and Obstetric Anal Sphincter Injury Among Unassisted Vaginal Births: A Retrospective Cohort Study

Mediolateral episiotomy (MLE) is a common procedure. Whether it modifies the risk of obstetric anal sphincter injury (OASI) in nulliparous and parous people is unclear.

We performed a retrospective cohort study to assess if MLE was associated with decreased OASI.

A retrospective cohort study including all unassisted vaginal births from 36 weeks' from 2008 to 2021 was assembled. Analysis was separated for first and subsequent vaginal births, adjusting for maternal height, body mass index (BMI), ethnicity, prior caesarean birth (if no previous vaginal birth) or pre-delivery parity (if there was a previous vaginal birth), smoking status, augmentation of labour, induction of labour, length of second stage, caregiver type, socioeconomic deprivation quintile, gestational age, baby head circumference, baby birthweight and use of epidural using Generalized Linear Models.

There were 49 363 unassisted vaginal births. Of these, 20 201 were first vaginal births (19 082 nullipara and 1119 with one prior caesarean birth), 27 825 subsequent vaginal births, and 1337 unknown vaginal birth status. Episiotomy and OASI were recorded in 28.6% and 3.6% of first and 9.1% and 0.8% of subsequent vaginal births, respectively. Among first vaginal births, there was no association between episiotomy and OASI in univariable (RR 1.00 (0.86, 1.17)) or multivariable analysis (RR 0.91 (0.77, 1.09)). Among subsequent vaginal births, episiotomy was associated with increased odds of OASI in univariable (RR 2.21 (1.57, 3.13)) and multivariable analysis (RR 1.55 (1.08, 2.22)).

Our findings do not support the use of MLE for first births and suggest harm for subsequent births.