Blood clots in the legs are a common and potentially dangerous health problem. Doctors call this condition deep vein thrombosis, or DVT. It happens when a blood clot forms in one of the deep veins, usually in the calf or thigh.
While some clots remain in the leg, others can break away and travel to the lungs, causing a pulmonary embolism. This medical emergency can be life-threatening, making it important to recognise the causes, symptoms and risk factors early.
Blood clotting is a normal process that helps stop bleeding after an injury. However, clots that form inside healthy veins without a good reason can block blood flow and damage nearby tissues.
Research over many decades has shown that DVT usually develops when blood flow slows down, the blood becomes more likely to clot, or the blood vessel wall is damaged. These three factors are often known as Virchow’s triad and remain the foundation for understanding why blood clots develop.
One of the biggest risk factors is staying still for long periods. Long-distance flights, extended car trips, prolonged bed rest and recovery after surgery all reduce blood flow through the leg veins. Studies have found that regular movement, leg exercises and walking whenever possible help lower this risk by improving circulation.
Major surgery and serious injuries also increase the chance of developing a clot. Operations involving the hip, knee or pelvis carry particularly high risk because they damage tissues and often require patients to spend time resting in bed.
Hospitals routinely use blood-thinning medicines and compression stockings because research has consistently shown they reduce the risk of DVT after surgery.
Cancer is another important cause. Many cancers make the blood more likely to clot, and some cancer treatments further increase the risk. Research shows that people with cancer are several times more likely to develop DVT than the general population. Doctors often pay close attention to clot prevention in these patients.
Age is another well-established risk factor. Blood clots become more common as people grow older because circulation changes and other health conditions become more frequent. Although DVT can occur at any age, the risk rises steadily after the age of 60.
Obesity also increases the likelihood of blood clots. Excess body weight places additional pressure on the veins and is often linked with lower physical activity. Large population studies have found that maintaining a healthy weight helps reduce the risk of DVT and many other cardiovascular diseases.
Smoking is another factor supported by research. Chemicals in cigarette smoke damage blood vessels and make blood more likely to clot. Quitting smoking lowers the risk of many heart and blood vessel diseases, including DVT.
Pregnancy and the weeks after childbirth are periods when blood clotting naturally increases to protect against bleeding during delivery. However, this protective change also raises the risk of DVT, particularly in women with other risk factors such as obesity or a family history of blood clots.
Some people inherit genetic conditions that make their blood clot more easily. These inherited disorders, such as Factor V Leiden mutation, are less common but can significantly increase risk, especially when combined with surgery, pregnancy or prolonged inactivity.
Certain medicines also play a role. Birth control pills and hormone replacement therapy containing oestrogen have been linked with a higher risk of blood clots, although the absolute risk remains low for most healthy people. Doctors carefully weigh the benefits and risks before prescribing these treatments.
The most common symptoms of DVT include swelling in one leg, pain or tenderness, warmth, redness or changes in skin colour. However, some people have very few symptoms, making diagnosis difficult. Ultrasound scanning is now the main test used to confirm the condition because it is accurate and non-invasive.
Research has shown that prompt treatment is highly effective. Blood-thinning medicines, known as anticoagulants, reduce the chance of the clot growing larger and help prevent dangerous pulmonary embolism. Some patients may also need compression stockings or other treatments depending on their individual circumstances.
Scientists continue to study better ways to predict, prevent and treat blood clots. Recent research focuses on identifying people at highest risk, improving blood-thinning medicines and reducing bleeding complications. Advances in these areas have already lowered deaths from DVT over recent decades.
Overall, research shows that most blood clots develop because several risk factors occur together rather than from a single cause.
Staying physically active, maintaining a healthy weight, avoiding smoking, following medical advice after surgery and recognising early symptoms are among the most effective ways to reduce the risk. Early diagnosis and treatment remain the best protection against serious complications.
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