Introduction Obstetric anal sphincter injury (OASIS) occurs in 5%-7% of normal deliveries and increases with vacuum extraction (VE) to 12%-14% in nulliparous women in Sweden. Lateral/mediolateral episiotomy may reduce the prevalence of OASIS at VE in nulliparous women. The current use of episiotomy is restrictive. The protective effect and consequences are uncertain. This trial will investigate if lateral episiotomy can reduce the prevalence of OASIS and assess short-term and long-term effects. Methods and analysis This is a multicentre randomised controlled trial of lateral episiotomy versus no episiotomy in nulliparous women with a singleton, live fetus, after gestational week 34+0 with indication for VE. A lateral episiotomy of 4 cm is cut at crowning, 1-3 cm from the midline, at a 60° angle. The primary outcome is OASIS by clinical diagnosis analysed according to intention to treat. To demonstrate a 50% reduction in OASIS prevalence (from 12.4% to 6.2%), 710 women will be randomised at a 1:1 ratio. Secondary outcomes are pain, blood loss, other perineal injuries, perineal complications, Apgar score, cord pH and neonatal complications. Web-based questionnaires at baseline, 2 months, 1 and 5 years will be used to assess pain, incontinence, prolapse, sexual function, quality of life and childbirth experience. A subset of women will receive follow-up by pelvic floor sonography and pelvic examination. Mode of delivery and recurrence of OASIS/episiotomy in subsequent pregnancies will be assessed at 5 and 10 years using register data. Ethics and dissemination The trial is open for enrolment. The trial has received ethical approval from the Regional Ethical Review Board of Stockholm and full funding from the Swedish Research Council. Women are interested in participation. The predominant restrictive view on episiotomy may limit recruitment. Results are of global interest and will be disseminated in peer-reviewed journals and at international congresses
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