Obstetric anal sphincter injury after episiotomy in vacuum extraction: an epidemiological study using an emulated...

To emulate a randomised controlled trial investigating whether lateral or mediolateral episiotomy compared with no episiotomy reduces the prevalence of obstetric anal sphincter injury (OASIS) in nulliparous women delivered with vacuum extraction.

A population-based observational study.

Sweden.

63 654 nulliparous women delivered with vacuum extraction derived from the Swedish Medical Birth Register 2000-2011, with a live singleton baby with no known malformations in cephalic presentation in gestational week ≥34⁺⁰ , and subject to lateral or mediolateral episiotomy or no episiotomy.

The effect of episiotomy was calculated using a causal doubly robust estimation method based on propensity scores. Results are presented as the average treatment effect and numbers needed to treat (NNT).

OASIS (third- and fourth-degree perineal injury) in nulliparous women delivered with vacuum extraction.

Episiotomy was associated with a reduction in OASIS from 15.5% to 11.8%, average treatment effect of -3.66% (95% CI -4.31 to -3.01) and NNT 27. Third-degree perineal injuries were reduced from 14.0% to 10.9% (-3.08, 95% CI -3.71 to -2.42) with NNT 32. Fourth-degree perineal injuries were reduced from 1.6% to 1.0 % (-0.58%, 95% CI -0.79 to -0.37) with NNT 172.

Lateral or mediolateral episiotomy reduced the prevalence of OASIS in nulliparous women delivered with vacuum extraction, compared to women with no episiotomy.

To prevent one case of OASIS in first-time mothers delivered with vacuum, 27 episiotomies had to be performed.