Are women having a vaginal birth after a previous caesarean delivery at increased risk of anal sphincter injury?

OBJECTIVE

To examine the incidence of obstetric anal sphincter injury in women who had a successful vaginal birth after a previous caesarean delivery (VBAC).

DESIGN

Retrospective analysis of prospectively gathered data.

SETTING

A tertiary referral university institution.

POPULATION: All secundiparous women with a previous caesarean delivery who had a VBAC from 2001 to 2011.

METHODS

Details of maternal demographics, intrapartum characteristics and outcomes were examined in cases of VBAC with accompanying anal sphincter injury.

MAIN OUTCOME MEASURES: Rates of obstetric anal sphincter injury and associated risk factors.

RESULTS

During the study period there were 3071 trials of labour in secundiparous women with a previous caesarean delivery; 65% (1981/3071) of these had a successful VBAC. Women having a VBAC were at greater risk of anal sphincter injury than nulliparous women having a vaginal delivery over the same period (5% [98/1981] versus 3.5% [1216/34,496], P = 0.001, odds ratio 1.4, 95% CI 1.15-1.75). The rate of instrumental delivery in woman having a VBAC was 39% (771/1981). On multiple logistic regression analysis an increased rate of instrumental delivery was a strong predictor of sphincter injury (P = 0.03, odds ratio 1.15, 95% CI 1.01-1.3). When the first labours of women with sphincter injury in the VBAC group were examined, 70% (60/86) had been in labour before undergoing their caesarean delivery.

CONCLUSION

The incidence of anal sphincter injury in women undergoing VBAC is 5% and birthweight is the strongest predictor of this. The rate of instrumental delivery in this group was also increased.