Thrombophilia is a tendency for blood to clot too readily, which leads to an increased risk of disorders, such as deep-vein thrombosis and stroke, due to blockage of blood vessels by blood clots (called thrombosis). This tendency may be due to an inherited deficiency or abnormality in proteins such as factor V that are involved in blood clotting. Thrombophilia may also be caused by the immune disorder antiphospholipid syndrome, which is associated with an increased risk of miscarriage as well as of circulatory disorders due to thrombosis.

Thrombophilia may go unrecognized until conditions, such as air travel or injury, that increase the risk of clots cause signs or symptoms. Women with thrombophilia are advised not to take the oral contraceptive pill or hormone replacement therapy.