Parity-stratified risk factors for obstetric anal sphincter injuries: A retrospective cohort study

Obstetric anal sphincter injuries (OASIS) are among the most serious complications of vaginal delivery. While various prediction models have been proposed, most fail to stratify risk by parity and commonly rely on birth weight, a parameter unavailable during labor, rather than estimated fetal weight (EFW). The aim of the present study was to identify maternal and intrapartum risk factors for OASIS in term vaginal births and to explore parity-specific associations.

This retrospective cohort study included all women who achieved or intended to achieve vaginal delivery of a live, singleton, cephalic fetus at term (≥37 weeks) at a single tertiary center. Maternal, obstetric, and intrapartum data were extracted from electronic medical records. EFW was usually obtained sonographically, or clinically when no recent ultrasound was available. Multivariable logistic regression analyses were performed for the whole cohort and stratified by parity.

The overall rate of OASIS was 0.46% (102/22220) for the entire cohort. Stratified by parity, the prevalence was higher in nulliparous women compared to multiparous women (67/7309; 0.92% for nulliparas vs. 35/14911; 0.23% for multiparas). Overall, nulliparity, vacuum-assisted delivery, and increased EFW increased the risk for OASIS. However, following stratification by parity and adjustment for confounders, the only statistically significant risk factor among nulliparous women was vacuum-assisted delivery (adjusted odds ratio [aOR]: 2.474, 95% confidence interval [CI]: 1.520-4.026, P < 0.001), while among multiparous women, previous cesarean delivery was associated with OASIS, (aOR: 2.018, 95% CI: 0.507-8.024, P = 0.013). Neither EFW nor second-stage duration remained significant in stratified models.

Traditional indicators such as second-stage duration and EFW did not reliably identify women at risk of OASIS in this cohort. The strongest associated factors were first-time vaginal birth and vacuum-assisted delivery, with previous cesarean delivery relevant among multiparous women. These findings highlight clear parity-related patterns that merit further investigation.