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ObjectiveTo evaluate and compare the risk profile of sustaining obstetric anal sphincter injuries (OASIS) and associated risks in five risk groups (low to high), after the OASIS rate was reduced from 4.6% to 2.0% following an interventional programme. The main focus of the intervention was on manual assistance during the final part of second stage of labour.

DesignA multicentre interventional cohort study with before and after comparison.

SettingFour Norwegian obstetric departments.

SampleA total of 40154 vaginal deliveries in 2003-09.

MethodsPre-intervention and postintervention analyses. The associations of OASIS with possible risk factors were estimated using odds ratios obtained by logistic regression.

Main outcome measureRisk factors of OASIS.

ResultsThe risk of sustaining OASIS decreased by 59% (odds ratio [OR] 0.41; 95% confidence interval [95% CI] 0.36-0.46) after the intervention. Associations with obstetric risks for OASIS were largely unchanged after the intervention, including first vaginal delivery (OR 3.84; 95% CI 2.90-5.07), birthweight 4500g (OR 4.42; 95% CI 2.68-7.27), forceps delivery (OR 3.54; 95% CI 1.99-6.29) and mediolateral episiotomy (OR 0.89; 95% CI 0.70-1.12). However, the highest reduction of OASIS, (65%), was observed in group 0 (low-risk) (OR 0.35; 95% CI 0.24-0.51), and a 57% (OR 0.43; 95% CI 0.35-0.52), 61% (OR 0.39; 95% CI 0.31-0.48), and 58% (OR 0.42; 95% CI 0.30-0.60) reduction in groups with one, two and three risk factors, respectively. No change was observed in the group with four risk factors.

ConclusionAfter the intervention the most significant decrease of OASIS was observed in low-risk births, although the main risk factors for OASIS remained unchanged.