Hereditary thrombophilia, such as that associated factor V Leiden and the prothrombin G20210A gene variant, has been reported in Caucasians, but such findings have not been reported in Asians.8) Conversely, protein S deficiency is more common in Japan than in other East Asian countries because of the protein S K196E mutation.9,10) In Japan, several case reports on DVT associated with pregnancy have been published, but there have been no comprehensive studies. This evidence concerns the gene PROS1 and inherited thrombophilia.