Obstetric anal sphincter injuries (OASIS) encompass both third- and fourth-degree perineal tears [1]. A third-degree tear involves injury to the perineum extending into the external and/or internal anal sphincter complex. A fourth-degree tear includes disruption of the anal sphincter complex and extends through the anorectal mucosa [1]. These injuries are associated with long-term complications including anal incontinence and pelvic floor dysfunction [2]. This study aimed to evaluate the incidence of OASIS and identify associated maternal, fetal, and intrapartum risk factors in a large UK tertiary level centre.

This was a retrospective cohort study of all vaginal births at a UK tertiary hospital between January 2018 and December 2022. OASIS was defined in line with RCOG guidelines [1]. Data collected included parity, gestational age, mode of delivery (spontaneous, vacuum-assisted, forceps-assisted), use of mediolateral episiotomy, maternal birthing position, and birth weight.

Descriptive statistics were used to characterize the population. Associations between categorical variables and OASIS were assessed using chi-square tests. Multivariable logistic regression was performed to identify independent predictors. Odds ratios (OR), 95% confidence intervals (CI), and p-values were reported; p < 0.05 was considered statistically significant

Of 15,703 births during the study period, 10,156 were vaginal deliveries. A total of 278 cases of OASIS were identified, representing an incidence of 2.74%, consistent with national figures [3].

OASIS occurred more frequently in forceps-assisted births (p < 0.0001). Compared to forceps, spontaneous vaginal (OR 0.20) and vacuum-assisted births (OR 0.25) had significantly lower odds of OASIS (p < 0.001 for both). Episiotomy was associated with OASIS on univariate analysis (p = 0.00044), but was protective in multivariable regression (OR 0.25; 95% CI: 0.16–0.41; p < 0.001), likely reflecting selective use in high-risk scenarios.

Non-upright positions (lithotomy, semi-recumbent) were linked to higher OASIS risk (p < 0.0001), while upright positions (squatting, kneeling, all fours) appeared protective. Mean birth weight in OASIS cases was significantly higher (3,397g ± 466g), and remained predictive (OR 1.001; p < 0.001).

Nulliparity or low parity was strongly associated with OASIS (p < 0.0001). Higher parity was protective (OR 0.20; 95% CI: 0.15–0.27; p < 0.001). While gestational age > 41 weeks showed a trend toward increased risk, it did not reach significance in the final model

This five-year review confirms that OASIS is significantly associated with forceps-assisted delivery, higher infant birth weight, low parity, and non-upright birthing positions. Episiotomy appears protective in adjusted models, likely due to selective clinical use. These findings are consistent with recent evidence from 2023–2024 and support targeted perineal protection strategies during childbirth [3,4,5]

With an incidence of 2.74%, OASIS remains a clinically significant concern. Risk-reduction strategies should priorities reducing unnecessary instrumental births—especially forceps—promoting upright maternal positions, and using mediolateral episiotomy selectively in high-risk cases to protect long-term maternal pelvic floor function.

Royal College of Obstetricians and Gynaecologists. The Management of Third- and Fourth-degree Perineal Tears. Green-top Guideline No. 29. London: RCOG; 2015
Andrews V, et al. Outcomes after repair of obstetric anal sphincter injuries: observational study. BMJ. 2008;336(7635):753–6
Gurol-Urganci I, Cromwell DA, Edozien LC, et al. Third- and fourth-degree perineal tears among primiparous women in England between 2000 and 2012: time trends and risk factors. BJOG. 2013;120(12):1516–25.