Antithrombin (AT) is an important regulator of the coagulation cascade because of its ability to efficiently inhibit proteases such as Factor (F) Xa and thrombin. Type I hereditary AT deficiency is characterized by a quantitative deficiency in the antigen and activity of AT to about 50% of normal. Type II hereditary AT deficiency is characterized by a normal antigenic level of AT, with a low level of activity due to a dysfunctional protein. Impaired synthesis, consumptive coagulopathy including pregnancy-induced AT deficiency in multiple pregnancies, and urinary protein loss are associated with acquired AT deficiencies. Inherited thrombophilias are the leading cause of maternal thromboembolism and are associated with increased risk of second- and third-trimester fetal loss, abruptions, severe intrauterine growth restriction, and early-onset severe preeclampsia. Among thrombophilias, AT deficiency has long been associated with a significant thrombotic tendency throughout gestation and the puerperium. Treatment for this disorder includes antithrombotic therapy with unfractionated heparin or low molecular weight heparin, followed by an oral vitamin K antagonist, such as warfarin. Some patients with very low AT levels may be resistant to heparin therapy and may require increased doses of heparin or AT concentrates. In addition, an acquired decrease of AT plasma levels is a common finding in patients with preeclampsia. It is suggested that the administration of AT concentrates improves uteroplacental circulation and influence the pathophysiology of preeclampsia. Furthermore, it has been demonstrated that hereditary AT deficiency is associated with fetal loss. In women with a severe thrombotic tendency and recurrent fetal loss, thromboprophylaxis may offer more benefits.