Low dose heparin prophylaxis reduces the frequency of postoperative calf vein thrombosis, popliteal and femoral vein thrombosis and nonfatal pulmonary embolism in patients undergoing elective abdominal surgery. In a recent study, it produced an eight-fold reduction in the frequency of major pulmonary emboli detected at autopsy in general surgical patients. Low dose heparin prophylaxis has been less effective in reducing the incidence of venous thromboembolism in patients undergoing hip surgery; in some studies this has been associated with an increased risk of bleeding.