Episiotomy and Obstetric Anal Sphincter Injury in the Setting of Instrumental Vaginal Birth: A Retrospective Cohort Study

BACKGROUND: Episiotomy may be performed during an instrumental vaginal birth. The evidence that the procedure is associated with less obstetric anal sphincter injuries (OASI) in this setting is scarce. AIMS: We undertook a retrospective cohort study to ascertain whether the performance of episiotomy was associated with fewer obstetric anal sphincter injuries in patients undergoing instrumental births over a 14-year period from one tertiary institution. MATERIALS AND METHODS: Every vaginal birth at Te Toka Tumai Auckland from gestational age 36 weeks was included in a retrospective cohort spanning the years 2008-2021. Forceps-assisted and Vacuum-Extraction and Sequential Instrumental births were included. Univariable analysis was performed for the forceps, Vacuum-extraction and Sequential Instrumental birth strata. Multivariable analysis was performed for all births using generalized linear models including maternal, neonatal and labour characteristics. RESULTS: Between 2008 and 2021 there were 3714 forceps-assisted births, 7793 Vacuum-extraction births and 505 Sequential Instrumental births. The majority of these births were carried out with the use of a mediolateral episiotomy (89%, 66%, and 93.5% respectively) and OASI occurred in 9.5%, 5.2%, and 16.2%. Among people having an instrumental birth (all groups forceps, vacuum-extraction and sequential instrumental births), mediolateral episiotomy was associated with lower relative risks of OASI after adjusting for type of instrument, parity and other clinical and demographic factors (RR 0.68 (0.58-0.80)). CONCLUSION: Mediolateral episiotomy was associated with a lower relative risk of OASI in patients having operative vaginal births.