Venous thromboembolism (VTE) occurs frequently in pregnant women and is a significant cause of maternal death. Hemostatic abnormalities were examined in 18 pregnant women with thrombosis. We studied five families with congenital antithrombin (AT) deficiency, and two families with congenital protein C (PC) deficiency. One woman with PC deficiency showed protein S (PS) Tokushima. The AT activity levels were significantly lower at the onset of thrombosis in the pregnant women than during the stable state. The PS activity and antigen levels were also significantly lower at the onset of thrombosis. In the patients with congenital AT deficiency, AT activity was significantly low in the stable state and decreased further at the onset of thrombosis. Although AT levels were normal before pregnancy, they subsequently decreased and in two cases the patients required the administration of AT after pregnancy. Gene analysis revealed one family with AT Budapest, one family with AT Toyama, and three families with AT Glasgow. Additionally, there were one family with PC Tochigi and one family with combined heterozygous of PC deficiency and PS Tokushima. In conclusion, the deficiency of natural anticoagulants, especially AT, is an important cause of pregnancy-related VTE.