When a first-time mother needs medical assistance during childbirth, the chance of obstetric anal sphincter injury--a serious kind of perineal trauma between the vulva and anus--is more than halved when a lateral episiotomy (angled incision) is made in the tissue between the vaginal and anal opening.

Researchers from Stockholm's Danderyd Hospital and Karolinska Institutet conducted a randomised clinical trial, the findings of which were reported in The BMJ.

Anal incontinence and trouble holding onto gas and excrement can result from obstetric anal sphincter injury (OASI), which affects the anal sphincter muscles. In addition, it can lower the quality of life and lead to sexual issues.

In Sweden, around 5% of all first-time women who give birth vaginally suffer from this severe type of perineal damage.

"Women giving birth for the first time and requiring instrumental birth are at the highest risk of sustaining OASI," said Sophia Brismar Wendel, associate professor in obstetrics and gynaecology at the Department of Clinical Sciences, Danderyd Hospital, Karolinska Institutet and senior consultant at the Department of Women's Health, Danderyd Hospital.

"Therefore, we wanted to assess if such injury could be prevented in this group with a lateral episiotomy (angled cut in the perineum) when the baby's head is crowning."

Participants were recruited to the EVA (Episiotomy in Vacuum Assisted Delivery) trial at eight Swedish hospitals between 2017 and 2023.

Over 6,100 pregnant women expecting their first child agreed to participate in the delivery required vacuum assistance. A total of 702 women with a vacuum-assisted delivery were randomly assigned to receive a lateral episiotomy or no episiotomy.

The lateral episiotomy reduced the risk of OASI by an average of 53%. Among the women assigned to the procedure, 6.1 per cent sustained an OASI, compared to 13.1 per cent in the comparison group.

Moreover, the procedure did not increase the risk of severe blood loss, negative birth experience, prolonged hospitalisation or additional complications. However, the risk of wound complications such as wound infection and wound dehiscence was increased in the episiotomy group.

Episiotomy as a medical intervention is controversial as it has historically also been used for normal births, sometimes without the woman's consent," said Sophia Brismar Wendel.

"The use of episiotomy is highly variable in different countries, and in Sweden, we've practised restrictive use given the lack of evidence. However, these results may impact obstetric care both in Sweden and elsewhere."

The trial also gathers data on the women's self-reported symptoms at one and five years after childbirth. The results from the one-year follow-up will soon be analysed, which might give some answers about any differences in symptoms, such as anal incontinence and sexual function.

"This may help doctors and pregnant women in shared informed decision-making regarding the use of a prophylactic lateral episiotomy," said Sophia Brismar Wendel.