Risk of obstetric anal sphincter injury among women who birth vaginally
after a prior caesarean section: A state-wide cohort study
Objective: Vaginal birth after caesarean (VBAC) has been suggested to be
associated with an increased risk of obstetric anal sphincter injury
(compared with primiparous women who birth vaginally). However, prior
studies have been small, or used outdated methodology. We set out to
validate whether the risk of obstetric anal sphincter injury among women
having their first VBAC is greater than that among primiparous women
having a vaginal birth. Design: State-wide retrospective cohort study.
Setting: Victoria, Australia. Population: All births (455,000) between
2009-2014. Methods: The risk of severe perineal injury between first
vaginal birth and first vaginal birth after previous caesarean section
was compared, after adjustment for potential confounding variables.
Covariates were examined using logistic regression for categorical data
and Wilcoxon rank-sum test for continuous data. Missing data were
handled using multiple imputation; the analysis was performed using
regression adjustment and Stata v16 multiple imputation and teffects
suites. Results: Women having a VBAC (n=5,429) were significantly more
likely than primiparous women (n=123,353) to sustain a 3rd or 4th degree
tear during vaginal birth (7.1 vs 5.7%, p<0.001). After
adjustment for mode of birth, body mass index, maternal age, infant
birthweight, episiotomy and epidural, there was a 21% increased risk of
severe perineal injury (relative risk 1.21 (95%CI 1.07 – 1.38)).
Conclusions: Women having their first vaginal birth after caesarean
section have a significant increased risk of sustaining a 3rd or 4th
degree tear, compared with primiparous women having a vaginal birth.
Patient counselling and professional guidelines should reflect this