Induction of labor and risk of postpartum hemorrhage in women with vaginal delivery: A propensity score analysis.

OBJECTIVE

To explore the association between induction of labor (IOL) and postpartum hemorrhage (PPH) after vaginal delivery.

METHODS

We included women from the merged database of three randomized prospective trials (TRACOR, CYTOCINON, and TRAAP) that measured postpartum blood loss precisely, with standardized methods. IOL was considered overall and according to its method. The association between IOL and PPH was tested by multivariate logistic regression modeling, adjusted for confounders, and by propensity score matching. The role of potential intermediate factors, i.e. estimated quantity of oxytocin administered during labor and operative vaginal delivery, was assessed with structural equation modeling.

RESULTS

Labor was induced for 1809 of the 9209 (19.6%) women. IOL was associated with a significantly higher risk of PPH of 500 mL or more (adjusted odds ratio 1.56, 95% confidence interval 1.42-1.70) and PPH of 1000 mL or more (adjusted odds ratio 1.51, 95% confidence interval 1.16-1.96). The risk of PPH increased similarly regardless of the method of induction. The results were similar after propensity score matching (odds ratio for PPH ≥500 mL 1.57, 95% confidence interval 1.33-1.87, odds ratio for PPH ≥1000 mL 1.57, 95% confidence interval 1.06-2.07). Structural equation modeling showed that 34% of this association was mediated by the quantity of oxytocin administered during labor and 1.3% by women who underwent operative vaginal delivery.

CONCLUSION

Among women with vaginal delivery, the risk of PPH is higher in those with IOL, regardless of its method, and after accounting for indication bias. The quantity of oxytocin administered during labor may explain one third of this association.