Investigate risk factors for obstetric anal sphincter injury (OASI) and their incidence between 2006 and 2010 at a regional teaching hospital in Australia. Determine whether a correlation exists between a decrease in OASI rates and an increase in mediolateral episiotomies.

Materials and Methods
A retrospective observational study conducted at the Geelong Hospital, Barwon Health, Victoria, Australia. Every adequately documented vaginal birth from 2006 to 2010 in the hospital was included (N = 7314, cases of OASI = 239). Patient data were obtained from the Barwon Health Birth Outcomes System database. Multinomial logistic regression, Pearson’s correlation coefficient and relative risk calculations were used for analysis.

Risk factors for OASI include nulliparity (OR 2.64, 95% CI 1.95–3.57, P < 0.01), instrumental delivery (OR 2.54, 95% CI 1.82–3.55, P < 0.01) and birth weight greater than 4 kg (OR 1.56, 95% CI 1.11–2.19, P = 0.01). There was a significant correlation between increasing mediolateral episiotomy use from 12.56% to 20.10% and a reduction in OASI rates over the 5‐year period (Pearson's correlation coefficient: −0.94, P = 0.02). The correlation remained when analysing normal vaginal births in isolation from instrumental (Pearson's correlation coefficient: −0.89, P = 0.04). Conclusions Clear risk factors for OASI include nulliparity, macrosomia and instrumental delivery. There was a significant correlation between increasing mediolateral episiotomy rates from 12.56% to 20.10% and decreasing OASI.