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

OBJECTIVE

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.

DESIGN

A population-based observational study.

SETTING

Sweden.

POPULATION: 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.

METHODS

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).

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

RESULTS

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.

CONCLUSIONS

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

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