Synthetic oxytocin prophylaxis vs physiologic third-stage care for postpartum hemorrhage prevention in physiologic births: A population-based cohort study (2015–2022)

OBJECTIVE: To describe third-stage care practices among midwives in Ontario, Canada and estimate the causal effect of synthetic oxytocin prophylaxis (synOT) vs physiologic third-stage care on postpartum hemorrhage (PPH) and severe PPH following physiologic birth. METHODS: This population-based cohort study included low-risk individuals in Ontario (2015-2022) with a physiologic birth who reached the third stage under midwifery care. Severe PPH was operationalized as PPH requiring transfusion. Adjusted relative risks (aRR) and 95% CI were estimated using modified Poisson regression with inverse probability of treatment weighting based on the propensity score. Weighted absolute risk differences were calculated to aid interpretation. RESULTS: Among 59,777 eligible births, 73.2% received synOT, 14.4% physiologic care, and 12.5% mixed management. Compared with physiologic care, synOT was associated with an effect estimate close to the null for PPH requiring transfusion in nulliparas (aRR 0.96, 95% CI 0.40-2.30; 0.1 fewer per 1000, 95% CI -1.6-3.6) and multiparas (aRR 0.97, 95% CI 0.48-1.97; 0.0 fewer per 1000, 95% CI -0.8-1.4). For PPH, synOT was associated with an increased risk, with greater imprecision among nulliparas (nulliparas: aRR 1.19, 95% CI 0.94-1.49; 7.5 more per 1000, 95% CI -2.2-19.8; multiparas: aRR 1.32, 95% CI 1.12-1.56; 8.7 more per 1000, 95% CI 3.2-15.2). CONCLUSION: Compared with physiologic third-stage care, synOT was not associated with clinically meaningful benefit for PPH or PPH requiring transfusion. For PPH, synOT was associated with increased risk. These findings do not support routine synOT prophylaxis in this low-risk population; further research is warranted to confirm these findings.