A case-control study of the relationship between a passive second stage of labor and obstetric anal sphincter injuries.

Objective: To estimate the relationship between a passive second stage of labor and obstetric anal sphincter injuries (OASIS).

Methods: A retrospective, case-control study was undertaken of women who delivered at a tertiary-care center in Chicago, IL, USA, between November 2005 and December 2012. Cases had sustained OASIS and were matched on the basis of parity with controls who had no OASIS. Data were obtained from an electronic repository and chart review. Participants with a passive second stage of labor lasting 60 minutes or more were deemed to have "labored down." A logistic regression model to predict OASIS was created.

Results: Overall, 1629 cases were compared with 1312 controls. OASIS were recorded among 1452 (57.8%) of 2510 women who did not labor down compared with 169 (40.0%) of 423 women who labored down (P<0.001). However, in binary logistic regression, the addition of laboring down to the model only increased the predictive accuracy from 80.1% to 80.7%.

Conclusion: When known risk factors for OASIS are accounted for, the effect of laboring down on perineal outcome is negligible.