Secular Trends in Recurrent Obstetric Anal Sphincter Injuries among 297,190 Women in Norway: A Cohort Study.

INTRODUCTION AND HYPOTHESES: A care bundle to prevent obstetric anal sphincter injuries (OASIS), including manual perineal protection, was launched in Norway in 2005. The study objective was to determine the secular trends of recurrent OASIS in second vaginal birth between 1999 and 2022. OASIS incidence among women without previous OASIS was analyzed for comparison.

METHODS

Data for this cohort study were obtained from the Medical Birth Registry of Norway. The study included 297,190 women with singleton pregnancies and two vaginal births during the study period. The outcome was OASIS incidence at second vaginal birth, separately for women with a history of OASIS (recurrent OASIS) and those without. To study the contribution of episiotomy, epidural analgesia, birth mode, birthweight, and maternal age to the secular trends of OASIS, logistic regression analyses with crude and adjusted odds ratios (aORs) with 95% confidence intervals (CIs) were determined.

RESULTS

Of the women with OASIS in their first vaginal birth, 5.4% (609 of 11,205) experienced recurrent OASIS in their second vaginal birth. The incidence of recurrent OASIS decreased from 7.7% in 1999-2002 to 3.5% in 2019-2022, representing a 51% reduction (95% CI 27-67%). Among women without previous OASIS, the OASIS incidence decreased from 1.6% in 1999-2002 to 0.6% in 2019-2022. Obstetric interventions or birthweight did not contribute to these reductions.

CONCLUSIONS

The incidence of recurrent OASIS halved in second vaginal births among women with a previous OASIS. This reduction is likely explained by the national care bundle to reduce OASIS, launched in Norway in 2005.