[1] It is a rare hereditary disorder that generally comes to light when a patient suffers recurrent venous thrombosis and pulmonary embolism, and repetitive intrauterine fetal death (IUFD).

[2] Hereditary antithrombin deficiency results in a state of increased coagulation which may lead to venous thrombosis.

[3] The prevalence of antithrombin deficiency is estimated at ~0.02 to 0.2% of the general population, and 1-5% of patients with venous thromboembolism.

[6] In patients with antithrombin deficiency, they may develop resistance to unfractionated heparin, especially with continuous infusions.

If large quantities of unfractionated heparin are required e.g. greater than 35000 units per day, this would point towards resistance.