Risk indicators for obstetrical anal sphincter injury in vaginal birth after caesarean section compared to first vaginal delivery

Objectives: Comparison of the rate of obstetric anal sphincter injury (OASI) between women having their first vaginal birth after caesarean section (CS) and true nulliparous women with a vaginal delivery. Assessment of risk indicators for OASI in women with vaginal birth after one CS (VBAC).Study design: 28 535 women with their first VBAC and a cohort of 275 439 nulliparous women with a vaginal delivery of a liveborn infant in a cephalic position from the Dutch perinatal registry were analyzed. We compared the OASI rate with univariate and multivariate analysis. In women with VBAC possible risk indicators for OASI were assessed using univariate and multivariate logistic regression analysis.Results: The rate of OASI was 5.2% in women with vaginal birth after CS and 4.0% in women with a first vaginal delivery. The adjusted OR (aOR) for vaginal birth after an elective CS was higher (aOR 1.34, 95% CI 1.23-1.47) compared to vaginal birth after an emergency CS (aOR 1.16, 95% CI 1.08-1.25). In women with vaginal birth after emergency CS, the aOR for the indication non-progressive labor was 1.18 (95% CI 1.08-1.29), whereas CS for suspected fetal distress was not significantly associated with obstetric anal sphincter injury in VBAC. In the 28 535 women with a VBAC, mediolateral episiotomy (MLE), birth weight < 3000 g and maternal age < 25 years were associated with a significantly lower rate of OASI. A gestational age of 42 weeks, birth weight = 3500 g, operative vaginal delivery and duration of the 2nd stage of labour of = 60 min were associated with a significantly higher rate of OASI.Conclusions: Women with a VBAC have a higher rate of OASI in comparison with women with a first vaginal delivery, with the exception of women with a vaginal birth after an emergency CS for suspected fetal distress. Factors associated with a significantly lower rate for OASI were MLE, maternal age < 25 and birth weight < 3000 g. A gestational age of 42 weeks, birth weight between 3500 and 4000 g and = 4000 g, operative vaginal delivery and duration of the 2nd stage of delivery longer dan 60 min were associated with a significantly higher rate of OASI.