Deep vein thrombosis. Swollen and inflamed right thigh (left on image) due to deep vein thrombosis (DVT) in a female patient. The left thigh is unaffected. Thrombosis is the formation of a thrombus (clot) in a vein which is usually caused by a combination of sluggish blood flow and a condition that increases the tendency of the blood to clot. This may be caused by obesity, pregnancy, oral contraceptives, injury, infection, liver disease or certain cancers. This condition is more common in post-menopausal women. Part of a clot may be carried to the lungs, heart or brain causing an embolism. Treatment includes using anti-coagulant drugs or surgery.