The Impact of Midwife Experience and Time of Birth on the Risk of Obstetric Anal Sphincter Injuries (OASIS)

Obstetric anal sphincter injuries (OASIS) are a significant complication of vaginal birth. While most studies focus on patient-related risk factors, the impact of midwife experience and shift timing remains understudied. We examined the association between these factors and OASIS risk in spontaneous vaginal deliveries.

This retrospective cohort study analyzed term, singleton, cephalic, spontaneous vaginal deliveries at the Galilee Medical Center (March 2020-February 2025), excluding operative vaginal births and cesarean deliveries. Deliveries were categorized by midwife experience: inexperienced (< 2 years), moderately experienced (2-10 years), and highly experienced (> 10 years). Shift timing and delivery patterns within shifts were also examined. The primary outcome was OASIS incidence. Multivariate logistic regression adjusted for confounders such as parity, macrosomia, and second-stage duration.

Among 13,651 term spontaneous vaginal deliveries, OASIS occurred in 0.6%, with the highest rate among early-career midwives (1.9%) compared to mid-career (0.6%) and experienced midwives (0.5%) (p < 0.001). Associations remained significant after multivariate adjustment. Early-career midwives had a higher risk of OASIS (aOR 3.29, 95% CI 1.49-7.26), while mid-career midwives had a comparable risk to experienced midwives (aOR 1.05, 95% CI 0.60-1.85). Evening shift deliveries (aOR 1.73, 95% CI 1.01-2.96) and deliveries during the last 2 h of a shift (aOR 1.82, 95% CI 1.05-3.14) were also independently associated with increased OASIS risk.

Patients delivered by experienced midwives had lower rates of OASIS. This protective effect was especially evident in deliveries without episiotomy. Strengthening midwifery training and optimizing staffing may help improve perineal outcomes.