Deep vein thrombosis (DVT) is a blood clot (thrombus) that develops in a deep vein, usually in the lower leg. Deep vein thrombosis can cause pain in the leg and can potentially lead to complications. About 1-3 people in 1000 develop a DVT each year in western countries, and its incidence in India is similar.
DVT usually develops in a deep vein in the leg. Deep veins pass through the center of the leg and are surrounded by the muscles.
When blood clots outside a blood vessel, this is a normal process, which protects the body against losing blood. If the blood clots inside a blood vessel however (as with DVT), this can be dangerous.
In most cases of DVT, the clots are small and do not cause any symptoms. The body is able to gradually break down the clot and there are no long-term effects.
Larger clots may partially or totally block the blood flow in the vein and cause symptoms such as swelling and pain in the calf muscle of the leg.
If any patient develops pain and swelling which can not be explained DVT should be considered as a possible cause and should be investigated.
Complications:
It is uncommon for DVT to cause any further problems but potential complications include the following.
Pulmonary embolism (PE) happens when a piece of the blood clot breaks off and travels in the bloodstream to become lodged in the lungs and block blood flow. This can happen hours or even days after the formation of a clot in the calf  

veins. It may cause chest pain and shortness of breath. Rarely patient may even collapse and die due massive pulmonary embolism. Urgent investigations and management under these circumstances may save life
Post thrombotic syndrome happens if a DVT damages the valves in the vein, so that instead of flowing upwards, the blood pools in the lower leg. This can result in pain, swelling and ulcers on the leg. Causes of DVT: DVT can be caused by a damaged vein or if the flow of blood slows down or stops. There are certain risk factors that cause DVT like
Age >40 years Past history of DVT Family history of DVT Immobility Obesity Recent surgery or an injury, especially to the hips or knees Pregnancy Cancer Contraceptive pill that contains estrogen Hormone replacement therapy There is evidence that long haul flights (flight lasting four hours or more) may increase the risk of developing DVT (Economy class syndrome) .The risk is a result of prolonged immobility, which can happen during any form of long distance travel, whether'by car, bus, train or air.