ObjectivesLow dose aspirin has been shown to reduce the risk of preterm pre‐eclampsia and it has been suggested that it should be recommended for all pregnancies. However, some studies have reported an association between low dose aspirin and an increased risk of bleeding complications in pregnancy. Our aim was to evaluate the risk of placental abruption and postpartum hemorrhage (PPH) in patients for whom their healthcare provider had recommended prophylactic aspirin.MethodsThis multicenter cohort study included 72,598 singleton births at 19 U.S. hospitals (between January 2019 and December 2021). Pregnancies complicated by placenta previa/accreta, births occurring at less than 24 weeks’ gestation, multiple pregnancies, or missing data for aspirin recommendation were excluded. Propensity scores were calculated using 20 features spanning sociodemographic factors, medical history, year, and hospital providing care. The association between low dose aspirin recommendation and placental abruption or postpartum hemorrhage was estimated by inverse probability weighting using the propensity scores.ResultsWe included 71,627 pregnancies in the final analysis. Aspirin was recommended to 6,677 (9.3%) and was more likely to be recommended for pregnant individuals who were older (p<0.001), had a higher body mass index (BMI) (p<0.001), had pre‐pregnancy hypertension (p<0.001), and who did not have a spontaneous vaginal birth (p<0.001). Overall, 1,205 (1.7%) of the study cohort developed preterm preeclampsia: 1.3% in the no aspirin and 5.7% in the aspirin group. After inverse probability weighting with propensity scores, aspirin was associated with increased risk of placental abruption (adjusted OR 1.44; 95% CI 1.04, 2.00) and postpartum hemorrhage (adjusted OR 1.21; 95% CI, 1.05,1.39). The ‘number needed to harm’ with low dose aspirin was 1 in 79 (95% CI 1 in 43, 1 in 330) for postpartum hemorrhage and 1 in 287 (95% CI 1 in 127, 1 in 3151) for placental abruption.ConclusionsLow dose aspirin recommendation in pregnancy was associated with increased risk for placental abruption and for postpartum hemorrhage. Our results support the need for more research into aspirin and bleeding complications in pregnancy before recommending it beyond the highest risk pregnancies.This article is protected by copyright. All rights reserved.