Objective: To identify the risks of sustaining obstetric anal sphincter injury (OASI) during childbirth.
Methods: Data were analysed from 12,612 vaginal deliveries recorded at Northwick Park District General Hospital, London, from 1 January 2006 to 30 November 2009.
Results: 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.
Conclusions: 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.