Association Between Active Phase of the Second Stage of Labour and Obstetrical Anal Sphincter Injury Risk: A Contemporary, Retrospective, Population-Based Cohort Study.

INTRODUCTION AND HYPOTHESIS: Although the association between the prolonged second stage of labour and obstetrical anal sphincter injury (OASI) risk has been established, the contribution of the duration of the active phase (time spent pushing) remains unclear. This study was aimed at estimating the association between the duration of the active phase of the second stage of labour and OASI risk among nulliparous pregnant persons.

METHODS

We conducted a population-based, retrospective cohort study of nulliparous pregnant persons who gave birth to a singleton, non-anomalous, vertex foetus at ≥ 37 weeks' gestational age from 2017 to 2021. Poisson regression models adjusting for confounders were developed to estimate the risk ratio (RR) with 95% confidence intervals (CI) for the association between duration of the active phase, and total duration of the second stage, of labour and OASI.

RESULTS

Of 4963 persons, 471 (9.5%) cohort members sustained an OASI (8.7% third-degree and 0.8% fourth-degree). An active phase ≥ 120 min was associated with a 92% higher risk of OASI compared with 30 to < 60 min (RR 1.92, 95% CI 1.51-2.45). When stratified by mode of delivery, RRs estimated for this association were attenuated (spontaneous: 0.92, 95% CI 0.56-1.53; vacuum: 1.37, 95% CI 0.77-2.45; forceps: 0.85, 95% CI 0.63-1.15). When total duration of the second stage was considered, a duration ≥ 120 min was associated with a 59% increased risk of OASI compared with 30 to < 60 min (1.59, 95% CI 1.17-2.16).

CONCLUSION

Increasing duration of the active phase of the second stage of labour is associated with an increased risk of OASI.