Parity and anal sphincter trauma.

Introduction and hypothesis: Obstetric anal sphincter injury (OASI) is a form of maternal trauma secondary to vaginal birth. Women with a history of OASI are deemed to be at a higher risk of recurrence. However, it is unclear if multiparity is associated with higher prevalence. The aim of the study was to test the following hypothesis: the prevalence of external anal sphincter (EAS) tears diagnosed on transperineal ultrasound (TPUS) is related to vaginal parity.

Methods: A retrospective observational study on 1,273 women who attended a tertiary urogynecology service between January 2014 and December 2016. Patients had undergone a standardized interview, clinical examination and 4D TPUS. Tomographic ultrasound imaging was used to evaluate the EAS, blinded to all clinical data.

Results: Of 1,273 women, 1,143 (90%) were vaginally parous, with a mean vaginal parity of 2 (range 1-8). Mean age was 54 (range 17-89) years. Anal incontinence was reported in 186 women (15%) with an average St Mark's incontinence score of 12 (range 1-23). EAS defects on ultrasound were identified in 148 (12%). On univariate analysis, there was no significant difference in the prevalence of EAS defects in vaginally primiparous and multiparous women (p = 0.460).

Conclusions: There is no significant difference in the prevalence of EAS tears between vaginally primiparous and multiparous women. This argues against additional traumatic effects of subsequent vaginal births.