Four pregnant women with antithrombin III congenital deficiency underwent thrombosis prophylaxis including oral anticoagulants administered from the 16-18th week to the 36-37th week of pregnancy, subcutaneous heparin before the 16-18th week and after the 36-37th week, and a single infusion of AT III concentrate in the peripartum period in order to obtain a minimal level of 0.8 U/ml of AT III functional activity. The level of circulating AT III after the concentrate infusion needs to be evaluated by functional methods, because of a consistent amount in the concentrates of inactive AT III immunoreactive material. No thrombotic or haemorrhagic complication occurred after starting prophylaxis in any woman either in any newborn.