 The rare forms of congenital thrombophilia are typically caused by a deficiency of natural anticoagulants . They are classified as " type I " and are more severe in their propensity to cause thrombosis . The main ones are antithrombin III deficiency , protein C deficiency and protein S deficiency . Milder rare congenital thrombophilias are factor XIII mutation and familial dysfibrinogenemia ( an abnormal fibrinogen ) . It is unclear whether congenital disorders of fibrinolysis ( the system that destroys clots ) are major contributors to thrombosis risk . Congenital deficiency of plasminogen , for instance , mainly causes eye symptoms and sometimes problems in other organs , but the link with thrombosis has been more uncertain . 
