Obstetric anal sphincter injuries in spontaneous vaginal births in nulliparous pregnant individuals: a 21-year cohort study based on real-world data.

Background: In 2005, the Norwegian Directorate of Health implemented a national care bundle of education and training on correct perineal protection during childbirth to reduce increasing incidence of obstetric anal sphincter injuries (OASIS). The intervention involved manual support of the perineum using one hand protecting the perineum, with the other hand controlling the expulsion speed of the baby's head, communicating with the individual in labor to avoid pushing during the last phase of fetal head crowning to reduce the risk of OASIS, and performing a lateral or mediolateral episiotomy with correct technique by indication when necessary.

Objective: To determine the secular trends of OASIS incidence in nulliparous individuals with singleton spontaneous vaginal births at ≥22 weeks´ gestation in Norway during 2002‒2022.

Study design: This was a population-based cohort study based on real-world data of 324,930 nulliparous pregnant individuals with singleton spontaneous vaginal births at ≥22 weeks gathered from Medical Birth Registry (MBR) of Norway. The main outcome measure was OASIS incidence during the 21-year time period from 2002 to 2022. Secular trends of OASIS were determined separately for the seven three-year time periods. Both descriptive statistics and multivariable analyses by logistic regression were performed. Both crude and adjusted odds ratios (aOR) with 95% confidence interval (CI) were determined.

Results: We observed that the OASIS incidence reduced significantly from 5.4% during 2002‒2004 to 2.1% during 2020‒2022. After adjustment for the individual´s age and country of birth, infant birthweight, use of epidural analgesia and episiotomy, the OASIS incidence was reduced by 63% (adjusted odds ratio = 0.37, 95% confidence interval = 0.34-0.40) from 2002‒2004 to 2020‒2022. This reduction in OASIS incidence was independent of individual characteristics, episiotomy use and infant birthweight. The use of epidural analgesia during birth accounted for 7.8-15.8% of the observed reduction in OASIS incidence between 2002 and 2016.

Conclusions: The substantial decrease in OASIS incidence among nulliparous individuals with singleton spontaneous vaginal births can be attributed to the widespread adoption of manual perineal protection techniques, following comprehensive staff training.