Obstetric anal sphincter injury risk factors and the role of the mediolateral episiotomy.

Aims: 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.

Results: 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.