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ObjectiveTo compare the incidence of obstetric anal sphincter injuries (OASIS) in two time periods, before and after implementing a training programme for improved perineal support aimed at reducing the incidence of obstetric anal sphincter injuries. The secondary aim was to study incidence of obstetric anal sphincter injuries in subgroups defined by risk factors for OASIS.DesignPopulation-based cohort study.SettingUniversity hospital setting in Oslo, Norway.ParticipantsTwo cohorts of all delivering women in the largest hospital in Norway during two time periods (2003–2005 and 2008–2010) were studied. After excluding caesarean sections and preterm deliveries (< week 32), the study population consisted of 31 709 deliveries, among which 907 women were identified with obstetric anal sphincter injury.Primary and secondary outcome measuresIncidence of OASIS in two time periods. Maternal, obstetrical and foetal risk factors for OASIS were collected from the hospital obstetric database. Univariate analyses and multivariate logistic regression analyses, presenting adjusted ODs for OASIS, were performed.ResultsThe OASIS incidence was significantly reduced by 50%, from 4% (591/14787) in the first time period to 1.9% (316/16 922) in the second. This reduction could not be explained by changes in population characteristics or OASIS risk factors during the study years. The reduction of incidence of OASIS between the two study periods was consistent across subgroups of women; regardless of parity, delivery method and infant birth weight.ConclusionsA marked reduction in the incidence of OASIS was observed in all studied subgroups of women after implementing the training programme for perineal protection. Further, this reduction could not be explained by the differences in patient characteristics across the study period. These findings indicate that the training programme with improved perineal protection markedly reduced the risk of OASIS.

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Seven episiotomy incisions are described in the literature, although only midline, mediolateral or lateral episiotomies are commonly used. Recent research has demonstrated variations in both site and direction of the incision, and differences between the angle of incision at the time of crowning of the fetal head and the angle of the scar once the wound has been repaired. We review this evidence and suggest that this variation may undermine the reliability of much published work. We suggest a standardised definition of each type of episiotomy to establish uniformity going forward, so that future studies are amenable to comparison and meta-analysis.