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Introduction and hypothesis Obstetric anal sphincter injuries (OASIs) are the leading cause of anal incontinence in women. Modification of various risk factors and anatomical considerations have been reported to reduce the rate of OASI.

Methods A PubMed search (1989-2014) of studies and systematic reviews on risk factors for OASI.

Results Perineal distension (stretching) of 170 % in the transverse direction and 40 % in the vertical direction occurs at crowning, leading to significant differences (15-30 degrees) between episiotomy incision angles and suture angles. Episiotomies incised at 60 degrees achieve suture angles of 43-50 degrees; those incised at 40 degrees result in a suture angle of 22 degrees. Episiotomies with suture angles too acute (< 30 degrees) and too lateral (> 60 degrees) are associated with an increased risk of OASI. Suture angles of 40-60 degrees are in the safe zone. Clinicians are poor at correctly estimating episiotomy angles on paper and in patients. Sutured episiotomies originating 10 mm away from the midline are associated with a lower rate of OASIs. Compared to spontaneous tears, episiotomies appear to be associated with a reduction in OASI risk by 40-50 %, whereas shorter perineal lengths, perineal oedema and instrumental deliveries are associated with a higher risk. Instrumental deliveries with mediolateral episiotomies are associated with a significantly lower OASI risk. Other preventative measures include warm perineal compresses and controlled delivery of the head.

Conclusions Relieving pressure on the central posterior perineum by an episiotomy and/or controlled delivery of the head should be important considerations in reducing the risk of OASI. Episiotomies should be performed 60 degrees from the midline. Prospective studies should evaluate elective episiotomies in women with a short perineal length and application of standardised digital perineal support.