Abstract
INTRODUCTION AND HYPOTHESIS: Obstetric anal sphincter injuries (OASIS) are more common in the primiparous population, especially during operative vaginal delivery (OVD). It would therefore be interesting to establish what factors influence the risk of OASIS when adjusting for the risk bias of OVD. METHODS: Retrospective analysis of prospectively collected data from the maternity database at University of Southampton NHS Foundation Trust of primiparous women sustaining OASIS during a singleton, term, cephalic, non-operative vaginal delivery between 2004 and 2015. A control comparison was made of women with otherwise identical birthing conditions but resulting with intact perinea, delivering between 2014 and 2015. Univariate and multivariate logistic regression compared maternal, intrapartum and neonatal factors of those sustaining OASIS with those with an intact perineum. RESULTS: Seven hundred fifty-six women sustaining an OASI met the criteria, and comparisons were made with a control population of 212. Those sustaining an OASI were significantly older (p < 0.001), more likely to be Asian (4.6-fold, p < 0.001) and had heavier babies, with a 3.6-fold greater proportion over 4 kg (p = 0.001). They were more likely to have delivered post-term and had longer second stages of labour (both p < 0.001). Epidural anaesthesia was associated with a 67% reduction in OASIS. CONCLUSIONS: These findings support previous research regarding risk factors for OASIS. By controlling for bias of risk associated with operative vaginal delivery, this revealed a potential protective effect of the use of regional anaesthesia.