Nulliparous women with a live singleton pregnancy in cephalic presentation past 34 gestational weeks will be randomized to lateral episiotomy or no episiotomy when operative vaginal delivery by vacuum extraction is indicated. Primary outcome is clinically diagnosed obstetric anal sphincter injury (OASIS) of any degree.

The study started at Danderyd Hospital 30 June 2017. Danderyd Hospital is a large teaching hospital affiliated to Karolinska Institutet in Stockholm, Sweden. South General Hospital in Stockholm, Uppsala University Hospital, Falun Hospital, Helsingborg Hospital, Växjö Hospital, Umeå University Hospital, and Gothenburg University Hospital Östra have joined the trial 2017-2019. The trial had included 717 women of planned 710 women by 15 February 2023.

Informed consent was collected from nulliparous women with a singelton, live fetus in cephalic presentation, with a planned vaginal delivery and without previous gynecological surgery for incontinence or genital prolapse. Women were approached at any time from gestational week 18 until delivery, unless they were in severe pain or discomfort, or there was not enough time to consider the information, or any other reason not to obtain consent. The informed consent forms are kept at the receiving research department and a note of consent/no consent was made in the obstetric medical record.

At indication for vacuum extraction, the patient's consent was confirmed verbally and allocation was made via sealed opaque envelopes on the vacuum extraction equipment mobile cart. Randomization was performed 1:1 in random permuted blocks generated electronically by an external organization (Karolinska Trial Alliance).

Lateral episiotomy was performed according to the intervention description. A reduction of OASIS from 12.4% to 6.2% can be detected with 80% power and less than 5% risk of alpha-error (p<0.05) with 355 women in each group using Chi-square test comparing two independent proportions in a two-sided test (3% missing outcome). A smaller reduction is clinically valuable, although the risk-benefit relationship between receiving a prophylactic episiotomy and the chance of an intact perineum may limit the feasibility of a larger trial in a setting with a restrictive episiotomy policy. We have obtained ethical approval to randomise a total of 1400 women, which enables us to detect a reduction in OASIS rate at VE from 12.4% to 7.8% (p<0.05). We planned and performed an interim analysis in order to exclude an unethically large difference (p<0.01) at 350 randomised women. The stopping criteria were not met. In 2019, we decided to aim for 710 randomised women due to slow recruitment.

Data was collected prospectively through Case Record Forms and the obstetric medical record Obstetrix (Cerner Sverige AB), which covers approximately 98% of deliveries with complete data, and the Swedish Pregnancy Register covering 98% of births in Sweden from 2015. Using a weblink, questionnaires from the national Obstetric Perineal Tear Register, are sent out electronically for the follow-up of study participants. In addition, questionnaires regarding birth satisfaction/experience (BSS-R and CEQ 2.0) are sent at 8 weeks after delivery. Questionnaires regarding sexual function (FSFI and FSDS) and quality of life (Euro-QoL-5D) at delivery and after 12 months.

A follow-up from registers regarding mode of delivery and prevalence of episiotomy and OASIS in subsequent births will be performed at 5 years and 10 years after the index birth.

A substudy takes place in Stockholm, Uppsala, and Helsingborg, where randomised women are examined at 6-12 months after delivery with 3D endovaginal and endoanal ultrasound to detect levator and sphincter injuries, measure episiotomy scars, establish POP-Q scores, and evaluate a new questionnaire for pelvic floor function after perineal tears.

Another substudy has taken place in Stockholm and Falun, where women who have been asked for consent (both affirmative and non-affirmative responders), as well as midwives who have tried to obtain consent, were invited to a qualitative interview study to explore the recruitment experience of women and midwives.

Inclusion Criteria:

• Nulliparous women,18 years old or more, able to give written informed consent in Swedish or English, with a live singleton pregnancy, in a cephalic presentation over 34 completed gestational weeks with a medical indication for vacuum extraction.

Exclusion Criteria:

• Previous vaginal or perineal surgery for incontinence or genital prolapse, any contra-indication to vacuum extraction (ie fetal disorder or malformation).

Anonymized data on the primary outcome will be shared with other researchers for a joint publication.

When results have been published (after 1 year).

Anonymized data on the primary outcome will be shared with other researchers for a joint publication.

The allocation will be masked during statistical analyses.