Antithrombin III (AT III) is a physiological inhibitor of coagulation. AT III deficit, whether congenital or acquired, results in a state of hypercoagulability characterized by recurring instances of venous thrombosis in young people. Although AT III levels normally change little during pregnancy, a deficit can be associated to the appearance of recurring thromboembolism and the need to perform cesarean section increases the risk. We report a cesarean section under general anesthesia in a patient with congenital AT III, reviewing the etiology and pathophysiology of this entity as well as its treatment.