Background: Estrogen-containing oral contraceptives (OCPs) increase the risk for thrombosis 3-4 fold in otherwise healthy young women, although the absolute thrombosis rate in young OCP users is only 0.05% per year (Trenor CC, Pediatrics. 2011;127: 347). In our prospective cohort database 70% of lower extremity venous thromboembolism in adolescent girls were associated with OCP use, usually of recent onset (<6months). OCPs are associated with acquired resistance to activated protein C (APC), an important thrombin regulatory pathway, with an increase in fibrinolysis (Douxfils J, Contraception, 2020;102:396). A causative role for estrogens in thrombus formation in young women is not completely understood.