If a blood clot forms in a deep leg vein, the blood pools and the vein becomes inflamed. Besides the pain of the venous inflammation, the complications of deep vein thrombosis are particularly serious: if parts of the blood clot become dislodged from the blockage, they can be swept into the lung via the blood. This causes a pulmonary embolism that can be potentially life-threatening because it can prevent blood from reaching your lungs. The danger of a pulmonary embolism is most prevalent in the first three to five days of a thrombosis.