Photograph of the rear of a person's legs, showing the gross swelling around the right knee and oedema (swelling) of the calf caused by a deep vein thrombosis (phlebothrombosis) in the right leg. Deep vein thromboses are encouraged by situations that encourage sluggish blood flow; 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, an obstruction in a blood vessel supplying a lung.