Legs have two sets of veins—superficial veins and the deep veins that drain blood from the leg muscles. A deep vein thrombosis (DVT) is a blood clot that forms in the veins deep in the leg, usually in the calf or thigh, although DVT can occur in other veins including the pelvis and rarely in the upper limbs. DVT can block the flow of blood partially or completely and cause many or no symptoms.
Most DVT forms in the calf and about one fifth will extend into the thigh. Doctors refer to DVT as distal or proximal; distal DVT refers to DVT of the calf, proximal DVT refers to DVT above the knee and above a major vein known as the popliteal vein. Proximal DVT is more serious than distal DVT because this DVT further up the leg is more likely to embolise and the emboli are potentially larger.
The figure shows a typical DVT in a deep vein of the leg.
In many 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. However, if the clot is large and blood flow is partially or totally blocked in the vein, the following symptoms can occur:
Each year venous thrombosis occurs in about one in 1000 people in developed countries . DVT became well known several years ago because of its association with flying for long periods (often called ‘traveller’s thrombosis’ or ‘economy class syndrome’), but the risk of getting a DVT is higher in hospital. Only about one in 6000 people who go on a long flight  (more than 4 hours) will develop DVT, but up to one in two people admitted to hospital can develop DVT if no steps to prevent DVT are taken.
DVT might not cause any further problems, but potential complications associated with DVT can be serious and unpleasant and include: