The PARTUM Trial: Postpartum Aspirin to Reduce Thromboembolism Undue Morbidity

Inclusion Criteria: * ONE (or more) First Order Criterion: 1. Known inherited thrombophilia diagnosed prior to enrolment, regardless of family history of VTE: i. Heterozygous factor V Leiden, or ii. Heterozygous prothrombin gene variant, or iii. Protein C deficiency, or iv. Protein S deficiency, and/or 2. Antepartum immobilization for ≥7 days. Immobilization is defined as bed rest with 90% of waking hours spent in bed at any time during the antepartum period AND/OR TWO (or more) Second Order Criteria: 1. Pre-pregnancy BMI ≥30 kg/m² 2. Smoking in the current pregnancy or within 3 months prior to pregnancy 3. Previous clinical history of superficial vein thrombosis 4. Preeclampsia 5. Current pregnancy ending in stillbirth (pregnancy loss \>20 weeks gestation) 6. Unplanned cesarean delivery (unplanned = not a scheduled cesarean delivery) 7. Small-for-gestational-age infant at time of delivery (\<3rd percentile adjusted for gestational age and sex) 8. Peripartum or postpartum infection (symptoms/signs of infection and documented fever and laboratory evidence of infection with positive blood cultures or an elevated white blood cell count based on local laboratory cutoffs) 9. Postpartum hemorrhage (≥1000 mL of blood loss, regardless of delivery mode) Exclusion Criteria: 1. More than 48 hours since delivery at the time of randomization 2. Received more than 1 dose of LMWH since delivery 3. Need for postpartum LMWH prophylaxis or systemic anticoagulation as judged by their physician and/or local investigator. May include but is not limited to: 1. Documented history of provoked or unprovoked VTE 2. Mechanical heart valve(s) 3. Known antiphospholipid syndrome (APS) 4. Known high-risk inherited thrombophilia i) Antithrombin deficiency, or ii) Homozygous factor V Leiden, or iii) Homozygous prothrombin gene mutation, or iv) More than 1 thrombophilia: any combination of 2 or more: factor V Leiden, prothrombin gene mutation, protein C deficiency, protein S deficiency 4. Need for postpartum ASA as judged by their physician and/or local investigator. May include but is not limited to: 1. Documented history of myocardial infarction 2. Documented history of ischemic stroke or transient ischemic attack (TIA) 5. Active bleeding, excluding normal vaginal bleeding, at the time of randomization 6. Known medical condition as judged by their physician and/or local investigator to be a contraindication to ASA or LMWH including known ASA or LMWH allergy 7. \<18 years of age 8. Unable or declined consent