Sometimes, thrombosis may develop because of the combination of an increase in the clotting factors within the blood and slowing of the normal blood circulation. The use of hormones, particularly estrogen, may predispose a person to blood clotting. Pregnancy and prolonged bed rest are both associated with venous thrombosis.
Symptoms vary according to whether a superficial or deep vein is involved.
The most common type of venous thrombosis is associated with superficial veins. With varicose veins, a combination of poor blood circulation, mild skin inflammation, and, sometimes, ulceration may cause thrombophlebitis of the leg. This may result in local tenderness and swelling of the vein, and redness and swelling of the nearby skin. Deep vein thrombosis in areas other than the leg may be impossible to detect, and the diagnosis of deep vein thrombosis may require special techniques such as X rays of the veins, ultrasound, and blood-clotting tests. The most serious complication occurs when a piece of the blood clot breaks off to form an embolus.