Excretion of Rivaroxaban in Human Breast Milk

Inclusion Criteria: 1. Postpartum within 6 weeks of delivery 2. Greater than 18 years old at expected date of delivery 3. English speaking 4. Hemodynamically stable without concern for ongoing blood loss 5. Non-breastfeeding 6. Or breastfeeding an infant born at or equal to 37 weeks' gestation, weighing \>2600g at birth and is not requiring intensive care 7. Meet inclusion criteria for prophylactic rivaroxaban dose: pre-pregnancy BMI \>/= 40 and/or any personal history of VTE. Or \>/= 2 of the following: pre-pregnancy BMI 30-39, immobility, pregnancy complications, cesarean delivery, hysterectomy 8. Or meet inclusion criteria for therapeutic rivaroxaban dose: have an indication for therapeutic anticoagulation per the American College of Obstetricians and Gynecologists' (ACOG) practice bulletin #196, or other indication per their primary team, and recommended for such anticoagulation per their primary practitioner. Exclusion Criteria: 1. Pregnant 2. Less than 18 years old at estimated date of delivery 3. Hemodynamic instability and/or concern for ongoing blood loss 4. Newly diagnosed deep vein thrombosis (DVT) or pulmonary embolism (PE) in the postpartum period 5. Abnormal maternal renal or liver function (creatinine clearance \< 30 mL/min and/or liver function tests greater than lab normal) 6. A contraindication to rivaroxaban: hypersensitivity to rivaroxaban; active pathological bleeding 7. Breastfeeding an infant admitted to neonatal intensive care unit, infant gestational age at birth \<37w0d or weighing \<2600g. 8. If all of the following are met: age less than 40, ambulating, body mass index (BMI) less than 30, no active history of malignancy. 9. If patient lacks indication for therapeutic anticoagulation in the postpartum period per the American College of Obstetricians and Gynecologists' (ACOG) practice bulletin #196