Background Amniotomy is a commonly used labour intervention with uncertain evidence. In a recent Norwegian study, amniotomy was identified as an unrecognized risk factor for severe perineal trauma (SPT), however, the sample size was small. The aim of the present study was therefore, to investigate whether amniotomy is a risk factor for SPT, and to examine whether the timing of performing amniotomy affects the occurrence of SPT.

Methods This nationwide register-based study included 477 321 women who gave birth in 2014–2020. Data were collected from the Swedish Pregnancy Register. Simple and multiple binary logistic regression analyses was used to investigate the association between amniotomy and SPT, adjusted for other risk factors.

Results A significant association between amniotomy and SPT was found in the simple logistic regression for both nulliparous [OR 1.33; 95% CI, 1.28-1.38] and multiparous women [OR 1.12; 95% CI, 1.03-1.22]. However, this association did not remain in the multiple logistic regression in neither nulliparous [OR 0.96; 95% CI, 0.87-1.07] nor multiparous women [OR 0.83; 95% CI, 0.67-1.03].  An increased duration between amniotomy and birth decreased the odds for SPT in nulliparous women (OR 0.97; 95% CI, 0.96-0.98).

Conclusions Performing amniotomy near the birth in nulliparous women entailed an increased risk for SPT, indicating the importance of a controlled and slow birth of the baby, to prevent SPT.

Potential impact This study adds to the body of knowledge of amniotomy and SPT. Awareness of risk factors for SPT is essential to improve obstetric management for women giving birth.

Ethics and conflicts of interest The study followed the principles of the Declaration of Helsinki (World Medical Association, 2018). Ethical approvals were given by the Swedish Ethical Review Authority (No. 2019–03626, 2020–04657 and 2023-04074-02).