Obstetric Anal Sphincter Injury: Insights from a Norwegian Cohort Study

A Norwegian cohort study found that amniotomy was not associated with an increased risk of obstetric anal sphincter injury (OASI) in a contemporary regional birth registry, challenging the common assumption that amniotomy increases intrapartum tissue trauma.

The dataset confirms that instrumental vaginal delivery—both vacuum extraction and forceps—and higher infant birth weight remain strong, established risk factors for OASI. These associations were consistent across parity strata.

Analysis showed instrumental delivery and larger birthweight were independently and robustly associated with higher OASI risk, with instrumental modalities demonstrating the strongest relative associations. Importantly, amniotomy did not emerge as a significant risk factor after multivariable adjustment, indicating that the association seen in unadjusted summaries did not persist in adjusted models. Consequently, in this cohort, instrument use and fetal size appear more relevant to perineal trauma risk than amniotomy.

Methods relied on a retrospective regional birth registry that included parity, mode of delivery, birth weight, amniotomy status, and other clinical covariates; investigators used multivariate logistic regression to adjust for confounding and estimate independent associations.

Limitations include the observational design, potential residual confounding, and local documentation practices—notably unclear recording of the indication for amniotomy—that restrict causal inference and generalizability.