Recent data suggest rising rates of postpartum hemorrhage (PPH) secondary to uterine atony in developed countries. Oxytocin (OT) use is associated with increased rates of PPH as a result of uterine atony. We sought to determine the effect of prolonged use or high doses of OT on risk of PPH and transfusion. This was a secondary analysis of a prospective cohort study of all women who had a vaginal delivery of a singleton gestation at ≥37 weeks gestation within a single institution from 2010-2014. Women with a prior cesarean delivery and with multiple gestations were excluded. The cohort was stratified by duration of OT exposure < or ≥15 hours from initiation to delivery (≥15 hours OT duration represented the 90th percentile) and by maximum dose of OT (<20mU/min vs ≥20mU/min). The primary outcomes were rates of PPH (estimated blood loss >500cc), severe PPH (estimated blood loss >1000cc), and need for blood transfusion. Multivariable logistic regression was used to account for confounders, including prolonged labor duration, obesity and nulliparity. Receiver operating characteristic (ROC) curves were used to examine the predictive value of OT duration and maximum dose on PPH. Among 4,202 patients, maximum OT dose ≥20mU/min or OT exposure ≥15hrs was associated with an increased risk of PPH (aOR 2.48, 95%CI 1.72-3.59 and aOR 2.10, 95%CI 1.34-3.29, respectively; Table 1). OT ≥15hrs was also associated with an increased risk of blood transfusion (aOR 2.62, 95%CI 1.08-6.34; Table 1). Although OT duration and maximum dose were modest predictors of PPH at best, OT duration was a better predictor of PPH than maximum OT dose (AUC 0.70 vs 0.66, p = 0.02; Graph 1). In the stratified analysis by low or high dose OT, OT exposure >15 hours was associated with PPH, severe PPH and need for blood transfusion only among women exposed to low dose OT (<20mU/min) but not among women exposed to high dose OT. Prolonged OT exposure during labor is associated with increased rates of PPH and transfusion following a vaginal delivery. This trend remains after controlling for prolonged labor duration and is strongest among women receiving lower doses of OT. This suggests that long exposure to low doses of OT may be an important factor to consider in active labor management as it may predict or contribute to PPH.View Large Image Figure ViewerDownload Hi-res image Download (PPT)