When you sit without moving your legs for several hours, the calf muscles remain relatively inactive. Blood may consequently move more slowly through the deep veins.
This is one reason prolonged travel is associated with an increased risk of DVT.
According to the CDC, sitting for more than four hours during travel can increase the risk of developing a blood clot. This applies to airplane journeys as well as long-distance travel by car, bus, or train.
The risk is generally greater when someone also has other contributing factors, such as recent surgery, obesity, pregnancy, cancer, or a previous history of blood clots.
It is important to understand that not everyone who takes a long flight will develop DVT. Nevertheless, prolonged immobility is a preventable risk factor worth addressing.
Prolonged bed rest
Spending extended periods in bed can significantly increase the likelihood of blood clots developing in the legs.
This can happen when someone is recovering from:
When the legs remain inactive, the calf muscles cannot assist blood circulation as effectively.
Hospitalized patients may also experience inflammation, dehydration, and changes in blood clotting, depending on their medical condition.
For these reasons, doctors frequently assess hospitalized patients for their risk of venous thromboembolism and may recommend preventive medication or mechanical devices.
Surgery
Surgery is an important risk factor for DVT, particularly procedures involving the hips, knees, pelvis, or lower limbs.
Several mechanisms contribute to this increased risk.
First, surgical procedures can damage blood vessels and surrounding tissues. Second, the body's natural response to surgery can temporarily make blood more likely to clot. Third, patients may spend several days recovering without their usual level of physical activity.
Orthopedic procedures, especially hip and knee replacement operations, are particularly associated with an increased risk of venous thrombosis.
The risk does not necessarily disappear immediately after leaving the hospital. Some patients remain at elevated risk for weeks following an operation.
Depending on the type of surgery and individual risk factors, doctors may prescribe anticoagulants, recommend mechanical compression devices, and encourage early movement.
Leg injuries and fractures
An injury to the leg can increase the risk of blood clots, even when the injury does not initially appear particularly serious.
Damage to blood vessels may activate the clotting process. Swelling and inflammation can also interfere with circulation.
The risk becomes more significant when an injury requires:
A broken leg or serious knee injury may therefore increase the risk of DVT through several different mechanisms.
Pregnancy and the postpartum period
Pregnancy naturally changes the body's blood-clotting system.
During pregnancy, the body becomes more inclined to form clots. This is partly an evolutionary protective mechanism that helps reduce excessive bleeding during childbirth.
However, these changes also increase the risk of venous thrombosis.
As the uterus grows, it can place additional pressure on veins in the pelvis, making it more difficult for blood to return from the lower extremities.
The risk can remain elevated after delivery, particularly during the first six weeks following childbirth.
Women who have undergone a cesarean section, experienced significant immobility, or have a personal or family history of blood clots may have additional risk factors.
Anyone experiencing unexplained leg swelling, pain, or tenderness during pregnancy or after delivery should seek prompt medical assessment.
Birth control pills and hormone replacement therapy
Some hormonal medications can increase the risk of developing blood clots.
Combined hormonal contraceptives containing estrogen may increase the blood's tendency to clot. Hormone replacement therapy can also contribute to venous thromboembolism risk.
The level of risk depends on several factors, including:
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The type of medication.
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The amount of estrogen involved.
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Age.
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Smoking status.
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Body weight.
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Personal or family history of DVT.
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Inherited clotting conditions.
This does not mean that everyone taking hormonal contraception will develop a blood clot. For most people, the absolute risk remains relatively low.
However, people with additional risk factors should discuss their options with a healthcare professional before starting or changing hormonal medication.
Obesity and excess body weight
Obesity is associated with a higher risk of DVT.
Excess body weight can increase pressure within the veins of the pelvis and legs, potentially making it more difficult for blood to circulate efficiently.
Obesity may also be associated with chronic low-grade inflammation and metabolic changes that influence the clotting system.
The risk can become more significant when obesity occurs alongside limited physical activity, surgery, smoking, or other medical conditions.
Maintaining a healthy weight through balanced nutrition and regular physical activity can support overall cardiovascular health and help reduce some of these risk factors.
Cancer and cancer treatment
Certain cancers increase the likelihood of blood clot formation.
Cancer cells can release substances that activate the body's clotting system. Some cancer treatments, including chemotherapy and certain hormonal therapies, can further increase the risk.
Hospitalization, surgery, central venous catheters, and reduced mobility may add to the problem.
People undergoing cancer treatment should discuss their individual risk of DVT with their oncology team, particularly if they develop new swelling or pain in one leg.
Heart disease and other chronic conditions
Certain chronic health conditions are associated with an increased risk of venous thrombosis.
These include:
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Heart failure.
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Inflammatory bowel disease.
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Some autoimmune and inflammatory conditions.
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Serious infections.
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Certain kidney disorders.
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Conditions that affect blood clotting.
For example, people with heart failure may have reduced blood circulation and limited physical activity.
Chronic inflammatory conditions can also affect the normal balance between clot formation and clot breakdown.
Smoking
Smoking damages blood vessels and affects normal cardiovascular function.
It can contribute to inflammation, vascular injury, and changes in blood clotting.
Although smoking is not the only factor involved in DVT, avoiding tobacco is an important part of reducing overall cardiovascular risk.
Inherited blood-clotting disorders
Some people inherit genetic changes that make their blood more likely to clot.
One example is factor V Leiden, a genetic variation that affects one of the proteins involved in blood coagulation.
Other inherited conditions include prothrombin gene mutations and deficiencies involving natural anticoagulant proteins such as protein C, protein S, and antithrombin.
A family history of unexplained blood clots, especially at a young age, may warrant discussion with a healthcare professional.
However, inherited clotting disorders do not automatically mean that someone will develop DVT. Risk often depends on the combination of genetic and environmental factors.
The Mayo Clinic identifies immobility, surgery, injury, pregnancy, hormonal medication, obesity, smoking, cancer, and inherited clotting conditions among the recognized risk factors for DVT.
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4. What Does a Blood Clot in the Leg Feel and Look Like?
A blood clot in a deep vein does not always produce obvious symptoms. In fact, some people have DVT without realizing that anything is wrong.
When symptoms do develop, they commonly affect one leg.
You may notice that one calf looks larger than the other, feels unusually warm, or becomes painful when walking or standing.
Some people describe the discomfort as a persistent cramp or a pulled muscle. Others experience tenderness, throbbing, or a heavy sensation in the affected leg.
The appearance of the skin may also change. It can become red, purple, bluish, or darker than the surrounding area, depending on the person's natural skin tone and the severity of the circulation problem.
However, there is no single appearance or sensation that can reliably confirm a blood clot.
A person with DVT may have only one symptom, several symptoms, or no noticeable symptoms at all.
Can you feel a blood clot moving in your leg?
Generally, you cannot feel a blood clot physically moving through your bloodstream.
If part of a clot breaks away and travels to the lungs, you may suddenly develop symptoms such as chest pain, shortness of breath, rapid breathing, or dizziness.
These symptoms are not a sensation of the clot moving through the leg. They are signs that a potentially serious complication may have developed.
