Implementation of manual perineal protection reduced OASIS incidence in first vaginal births after previous caesarean section

The study aims were to examine temporal changes in obstetric anal sphincter injury (OASIS) incidence among women undergoing their first vaginal birth after a previous cesarean section (CS) during 1999-2022, a period of progressive implementation of manual perineal protection in Norway, and to compare OASIS incidence with nulliparous women having their first vaginal birth. Data were obtained from the Medical Birth Registry of Norway. Two cohorts were formed: women with a first birth by CS and a subsequent vaginal birth (vaginal birth after cesarean, VBAC) (n = 36,014) and nulliparous women having their first vaginal birth (n = 468,195). OASIS incidence declined over time among women undergoing VBAC, from 8.7% in 1999-2002 to 4.0% in 2019-2022. A similar decline was observed among nulliparous women, from 6.5 to 2.7%. Changes in obstetric interventions and birthweight did not account for these reductions. Despite this decline, OASIS incidence remained higher in VBAC compared with nulliparous women (6.2% vs. 4.3%; OR 1.46, 95% CI 1.40-1.53). These findings suggest that although OASIS incidence has decreased over time during implementation of manual perineal protection, the higher incidence after VBAC may reflect differences in underlying obstetric risk profiles between the groups.