Incidence and Risk Factors of Obstetric Anal Sphincter Injury at a Tertiary Care Hospital in Thailand: A 10-Year Retrospective Cohort Study.

Introduction and hypothesis: Obstetric anal sphincter injury (OASI) is a severe complication of vaginal delivery leading to short- and long-term consequences affecting women's physical and psychological health. This study is aimed at evaluating the incidence and risk factors of OASI at a tertiary care centre over the past decade.

Methods: This retrospective cohort study included all singleton primiparous and multiparous women who delivered vaginally between 2014 and 2023. Women giving birth at < 24 weeks and whose child weighed < 500 g at birth, or with incomplete records, were excluded. Key maternal and obstetric factors were extracted and analysed using univariate and multivariate logistic regression.

Results: Among 9956 women, the overall incidence of OASI was 9.4% (third-degree 8.7% and fourth-degree 0.7%). The incidence of OASI declined from 9.8% to 6.1% in the past decade, coinciding with a reduction in episiotomy rates. Multivariate analysis identified forceps extraction (aOR 6.15, 95% CI 3.66-10.33), prolonged second stage of labour (aOR 2.47, 95% CI 2.00-3.05), labour augmentation (aOR 1.62, 95% CI 1.38-1.89), deliveries performed by a faculty member (aOR 1.25, 95% CI 1.03-1.51) and every 100-g increase in neonatal birthweight (aOR 1.09, 95% CI 1.08-1.11) as significant risk factors. Delivery without an episiotomy was a protective factor (aOR 0.41, 95% CI 0.30-0.56).

Conclusions: The high incidence of OASI at our institution highlights the impact of obstetric practices. The findings reinforce the need for restrictive episiotomy policies and targeted strategies to mitigate modifiable risk factors associated with OASI.