Congenital antithrombin deficiency is an inherited condition that increases the risk of abnormal blood clots. Normally, blood clots collect around wounds to stop bleeding, forming a scab. However, when a blood clot abnormally forms in the bloodstream it can travel through the body and block blood flow to important organs. A lowered level of antithrombin, an anti-clotting protein normally found in the bloodstream, influences the formation of abnormal blood clots in people with this condition. About half of people with congenital antithrombin have at least one abnormal clot during their life, and are particularly at risk for clots that occur deep in the veins of the leg (called deep vein thrombosis). If this type of clot becomes dislodged, it can become stuck in the blood vessels of the lungs, causing a serious condition called pulmonary embolism. Women with congenital antithrombin deficiency are at an increased risk of developing an abnormal clot during and soon-after pregnancy, and have an increased risk for miscarriage and stillbirth. The condition is inherited in an autosomal dominant pattern, meaning only one affected gene is needed to cause the condition. Although there is no cure for the condition, there are many ways to help lower the risk of blood clots and health emergencies caused by them, such as avoiding long periods of physical inactivity and having good exercise habits. Medications can also be used to decrease risks of blood clot formation. In addition, be aware of abnormal blood clot symptoms: swelling, redness, and/or pain in the affected area.