A thrombosis is a blockage of a blood vessel by a blood clot, often in the legs. Clots can develop as a response to damage or if there is insufficient blood flow. They can occur if a person suffers from a clotting disorder, is confined to bed, is seriously overweight or undergoes operations. Symptoms include numbness, pain, swelling and redness. If a blood clot breaks free from a deep vein thrombosis, there is a risk it will cause a pulmonary embolism.