If DVT isn't treated, it can cause complications, including inflammation of the vein (phlebitis), leg ulcers and pulmonary embolism.

It happens when the clot leaves the leg vein and travels through the blood vessel system to the lungs

Complications

Common complications of DVT include

Chronic venous insufficiency
Post-thrombotic syndrome
Pulmonary embolism
Much less commonly, acute massive DVT lead to phlegmasia alba dolens or phlegmasia cerulea dolens, both of which, unless promptly diagnosed and treated, can result in venous gangrene.

Pathophysiology is unclear, but edema may increase soft-tissue pressure beyond capillary perfusion pressures, resulting in tissue ischemia and venous gangrene.

Infection rarely develops in venous clots. 

Suppurative (septic) thrombophlebitis, a bacterial infection of a superficial peripheral vein, comprises infection and clotting and usually is caused by venous catheterization.

If DVT isn’t treated, it can cause complications, including inflammation of the vein (phlebitis), leg ulcers and pulmonary embolism.
Pulmonary embolism is the most serious complication of DVT. It happens when the clot leaves the leg vein and travels through the blood vessel system to the lungs. When the clot lodges in the lungs, it can block the main artery to the lungs or one of its major branches. When this happens, the lung tissue is starved of blood and oxygen.
It’s estimated that around one third of people who experience a major pulmonary embolism will die. Life-saving treatment for pulmonary embolism includes medicines to dissolve the clot and restore blood flow.

Pulmonary embolism
A pulmonary embolism (PE) occurs when a blood clot gets stuck in an artery in the lung, blocking blood flow to part of the lung. Blood clots most often start in the legs and travel up through the right side of the heart and into the lungs. This is called deep vein thrombosis (DVT).