Is the older perineum a safer perineum? Risk factors for obstetric anal sphincter injury.

INTRODUCTION

Obstetric anal sphincter injury (OASI) is the most common cause of anal incontinence. Identifying risk factors may facilitate change in labour and delivery practice, potentially reducing the risk. The objective of this study is to identify maternal, foetal and intrapartum risk factors for OASI in a regional hospital.

METHOD

We conducted a retrospective analysis of vaginal deliveries over a 10-year period (2008-2017). Anal sphincter injury was diagnosed by an experienced clinician and classified according to RCOG recommendations. A multiple logistic regression model was created using the presence of OASI as the dependent variable. Coefficients were adjusted for relevant maternal, foetal and intrapartum risk factors.

RESULTS

During the study period, there were 23,887 vaginal deliveries. Of these births, 18,550 were spontaneous (77.66%), 3746 vacuum-assisted (15.68%), 1196 forceps (5.01%) and 395 sequential instrumental deliveries (1.65%). The overall rate of OASI was 1.76%, with an upward trend seen in nulliparous mothers. Significant factors that increased the risk of OASI were nulliparity, Asian ethnicity, delivery by forceps or sequential instruments, and shoulder dystocia. Vacuum delivery did not significantly increase risk.

CONCLUSION

Maternal age ≥ 35 years confers a protective effect after adjusting for parity, birth weight and mode of delivery. Given the context of an ageing reproductive population, additional research is required to investigate the impact of maternal age on anal sphincter injury.