Impact of delivery indication on levator ani muscle avulsion in forceps- and vacuum-assisted deliveries: prospective cohort study.

Abstract

The primary aim was to assess the prevalence of levator ani muscle (LAM) avulsion after forceps- and vacuum-assisted deliveries, adjusting for indication for assisted vaginal delivery (AVD) and including spontaneous vaginal deliveries for comparison. The secondary aim was to compare neonatal injury between forceps and vacuum deliveries. This prospective observational cohort study was conducted at Haukeland University Hospital, Bergen, Norway. Primiparous women were recruited shortly after delivery, between June 2021 and April 2023, and LAM avulsion was assessed using transperineal ultrasound 9-12 months later. Delivery and neonatal data were collected from patient electronic health records. The risk of LAM avulsion was compared between forceps, vacuum and spontaneous vaginal deliveries using multivariable logistic regression analysis. Assisted deliveries were stratified by indication (fetal distress or protracted second stage of labor) and subgroup analyses were conducted for each indication, comparing forceps with vacuum delivery. A total of 736 women were examined using transperineal ultrasound at a median of 307 (interquartile range, 279-332) days after delivery. The 252 women who delivered by forceps had a higher prevalence of LAM avulsion (29.8%) compared with the 252 women who delivered by vacuum (17.5%) and the 232 women who underwent spontaneous vaginal delivery (11.2%). The adjusted odds ratio (aOR) of LAM avulsion after forceps vs vacuum delivery was 1.99 (95% CI, 1.26-3.18). After a protracted second stage of labor, the odds of LAM avulsion after forceps vs vacuum delivery were even greater (aOR, 3.09 (95% CI, 1.53-6.46)). Among those who underwent AVD for fetal distress, no significant difference in the odds of LAM avulsion was observed between forceps and vacuum deliveries (aOR, 1.63 (95% CI, 0.83-3.23)). The odds of neonatal trauma were lower after forceps compared with vacuum delivery (aOR, 0.49 (95% CI, 0.25-0.92)). There was no significant difference in the prevalence of LAM avulsion between forceps and vacuum deliveries when the indication for AVD was fetal distress, but the odds of LAM avulsion were 3-fold higher for forceps delivery when the indication was protracted second stage of labor. Forceps delivery was associated with a 51% reduction in the odds of neonatal injury compared with vacuum delivery, independent of indication for AVD. © 2025 International Society of Ultrasound in Obstetrics and Gynecology.