Importance It is unclear whether a higher dose (150-160 mg) or a lower dose (75 mg) of aspirin should be used to prevent preeclampsia. Objectives To compare the risk of preeclampsia and bleeding complications between women using 150 to 160 mg of aspirin and those using 75 mg of aspirin for preeclampsia prevention. Design, Setting, and ParticipantsThis nationwide cohort study included 13 828 women giving birth at 22 weeks' gestation or later in Sweden between January 2017 and December 2020 who used low dose aspirin (75-160 mg) during pregnancy. Data were analyzed from October to November 2023. Exposure The use of 150 to 160 mg or 75 mg of aspirin in pregnancy. Main Outcome and MeasuresThe main outcome was a preeclampsia diagnosis recorded in the maternal birth record at the time of hospital discharge. The main safety outcome was postpartum hemorrhage, defined as bleeding more than 1000 mL after delivery. Relative risks (RRs) and 95% CIs were estimated using a doubly robust inverse probability-weighted regression adjustment model controlling for background characteristics. Results In the total cohort of 13 828 women, the mean (SD) age was 33.0 (5.5) years and 3003 women (21.7%) were nulliparous. Of the women, 4687 (33.9%) were prescribed 150 to 160 mg of aspirin, and 9141 (66.1%) were prescribed 75 mg of aspirin. A total of 10 635 women (76.9%) had at least 2 dispensed prescriptions of low-dose aspirin. Among women using 150 to 160 mg of aspirin, 443 (9.5%) developed preeclampsia compared with 812 (8.9%) of those using 75 mg of aspirin (adjusted RR [aRR], 1.07; 95% CI, 0.93-1.24). Additionally, the risk of postpartum hemorrhage between the groups was similar, with 326 women (6.9%) using 150 to 160 mg of aspirin experiencing a postpartum hemorrhage compared with 581 (6.4%) in the 75-mg group (aRR, 1.08; 95% CI, 0.90-1.30). Conclusions and Relevance In this cohort study of 13 828 women, no difference was found in preeclampsia incidence or bleeding complications between those using 150 to 160 mg of aspirin vs 75 mg of aspirin during pregnancy for preeclampsia prevention. These findings suggest that either dose may be a reasonable choice when using aspirin to prevent preeclampsia. However, large randomized trials investigating aspirin dose in pregnancy are still needed.