Obstetric anal sphincter injury risk reduction: a retrospective observational analysis

To identify the risks of sustaining obstetric anal sphincter injury (OASI) during childbirth.

Data were analysed from 12,612 vaginal deliveries recorded at Northwick Park District General Hospital, London, from 1 January 2006 to 30 November 2009.

A total of 85.6% were spontaneous deliveries and 14.2% were instrument deliveries. The majority (64.5%) sustained some form of perineal damage, 3.7% being OASI. Logistic regression analyses revealed the risk factors for OASI to be Asian ethnicity [odds ratio (OR) 4.798, 95% confidence interval (CI) 2.998-7.679], a maternal age of >40 years (OR 2.722, 95% CI 1.315-5.636), higher foetal birth weight (>4500 g; OR 6.228, 95% CI 2.695-14.392), lower parity (para 0; OR 16.803, 95% CI 7.697-36.685), and instrumental delivery. Forceps delivery posed the greatest risk (OR 8.4, 95% CI 5.822-12.151). Not having an episiotomy increased the risk of OASI by five times compared with having one.

Risk factors for OASI include maternal age >40 years, higher foetal birth weight, lower parity, instrumental delivery, and Asian ethnicity. Mediolateral episiotomy appears to reduce the risk of OASI. Specific variables have been identified for incorporation into a risk-reduction strategy that could be introduced antenatally to evaluate and assess OASI risk.