The goal of this study was to determine risk factors for obstetric anal sphincter injury and whether any of them could be modified. This was a retrospective review of 2572 women (cases=1286; controls=1286) that took place over a 10 year period at a University teaching hospital. Maternal (Age, Parity, BMI and ethnicity), Obstetric (gestational age, assistance during delivery, episiotomy) and fetal (weight) risk factors were analyzed using logistic regression model presented as odds ratio (OR) with 95% confidence intervals (CI). Both univariate and multivariate analyses were conducted with outcome variables comparing cases and controls. Cases without instrumental deliveries were also compared to controls to exclude for the effect of assisted delivery. This study showed that in addition to instrumental delivery, primiparity (OR 9.8; CI 7.8-12.3), episiotomy (OR 8.6; CI 6.4-11.6), gestational age over 41 weeks (OR 1.5; CI 1.2-1.9), fetal weight over 4 kg (OR 3.2; CI 2.3-4.4) and Asian ethnicity (OR 1.9; CI 1.4-2.7) all had a strong association with OASI. A raised BMI over 30 seemed to have a protective effect (OR 0.4; CI 0.2-0.5).
The majority of risk factors related to OASI are non-modifiable although gestational age and episiotomy are modifiable risk factors.