OBJECTIVE: Data suggest type O blood has a higher propensity for hemorrhage, although the literature on postpartum hemorrhage (PPH) is mixed and does not separate by mode of delivery. Our objective was to assess the relationship between PPH and ABO blood type by mode of delivery. STUDY DESIGN: This is a retrospective cohort study of all singleton gestations delivered after 34 weeks identified by the PeriBank database of Baylor College of Medicine January 2011-March 2018. Exclusion criteria were sickle cell disease, use of anticoagulant medications other than Aspirin 81 mg, and multiple gestations. Data were abstracted from the PeriBank regarding demographics and delivery outcomes. Analyses were conducted separately for cesarean delivery (CD) and vaginal delivery (SVD). Significant demographic differences between groups were controlled for in multivariate logistical regression. Quantitative variables were analyzed with ANOVA and categorical variables with chi squared. The primary outcome was the rate of PPH by blood type (A, B, AB, and O), defined as blood loss >500 cc in SVD and >1000 cc in CD. RESULTS: 32,023 women met inclusion criteria (SVD 22,484 (70.2%), CD 9,539 (29.8%)). Significant demographics differences between blood types included age, parity, race, induction of labor, regional anesthesia, and thrombocytopenia in the SVD group. In the CD group, age, parity, and race were significantly different between blood types. There was no observed difference in the rate of PPH by blood type for those who delivered via SVD (p¼0.4), including when controlling for demographic differences in logistic regression (p¼0.2). In the CD group, there was a significantly higher rate of PPH in women with type O blood (5.2% type O vs 3.8% type A vs 4.4% type B vs 4.2% type AB, p¼0.035), including when controlling for demographic differences (p¼0.02). In both SVD and CD groups, there was no difference in any of the secondary outcomes including blood transfusions, hysterectomies, intrapartum D+C, and ICU admission. CONCLUSION: Type O blood may be an additional risk factor for PPH at the time of CD.