Photograph of a woman with a swollen left calf (right on image), caused by a deep vein thrombosis (blood clot) or phlebothrombosis: her right leg appears normal. Deep vein thromboses are provoked by situations that encourage sluggish blood flow and increase the tendency of the blood to coagulate; these include prolonged bed rest, surgery, heart failure and pregnancy. The main danger is that a part of the clot may become detached and cause a blockage in another part of the circulation - as in the case of a pulmonary embolism, where a blood clot is carried in the circulation to lodge in the pulmonary artery.