Why do blood clots form in leg veins?
Blood normally flows quickly through veins, and does not usually clot. Sometimes a DVT occurs for no apparent reason. However, the following increase the risk of having a DVT. • Immobility which causes blood flow in the veins to be slow. Slow flowing blood is more likely to clot than normal flowing blood. • A surgical operation which lasts more than 30 minutes is the most common cause of a DVT. The legs become still when you are under anaesthetic. Blood flow in the leg veins can become very slow. • Any illness or injury that causes immobility increases the risk of a DVT. • Long journeys by plane, train, etc are thought to cause a slightly increased risk of DVT. This is probably due to sitting cramped for long periods. • Faulty blood clotting is an uncommon cause. One example is an inherited condition that causes the blood to clot more easily than normal (factor V leiden). • The contraceptive pill and hormone replacement therapy (HRT) which contain oestrogen can cause the blood to clot
