Prevalence and Predictors of Anal Sphincter Injuries in Primipara with Vacuum-Assisted Deliveries

Women undergoing a vacuum-assisted (VA) delivery are at an increased risk of experiencing an obstetric anal sphincter injury (OASI). Studies using three-dimensional-endoanal ultrasound (3D-EAUS) have shown that not all OASIs are recognized immediately postpartum. In primipara with VA deliveries, the presence of and the potential clinical implications of undetected OASIs are undetermined and consequently, we examined prospectively a 1-year cohort of this population. We hypothesized that the total rate of OASIs would be higher than the clinically reported rates and wanted to identify potential associated predictors.

The participants were examined 10-14 days postpartum with 3D-EAUS. Two experienced examiners, blinded to the clinical data, rated the results. The primary outcome was the combined rate of clinically recognized OASIs and clinically unrecognized ultrasound anal sphincter injuries. Logistic regression analysis was used to calculate the odds ratios for selected variables.

Of 334 eligible women, 271 (81%) were included. The total rate of OASIs was 19% (95% CI 14-24; 14% clinically recognized and 5% clinically unrecognized). Prolonged second-stage/maternal exhaustion, occiput posterior presentation, and birthweight ≥ 4000g were significant and independent risk factors, increasing the odds two- to three-fold. The indication fetal distress alone or in combination with others significantly reduced the odds.

About 1 in 5 primipara undergoing a VA delivery experienced an OASI and about one quarter of them were not recognized clinically. The VA procedure is a modifiable intervention, and the identified risk and protective factors may help clinicians to decide whether it should be performed.