Deep vein thrombosis, or DVT, happens when a blood clot forms inside one of the large veins deep in your leg, usually in the calf or thigh. The underlying causes boil down to three conditions that have been recognized since the 1800s: sluggish blood flow, damage to the vein wall, and blood that clots too easily. Those three factors rarely act alone. In most cases, two or three overlap, and understanding how they interact explains why certain situations, from long flights to major surgery to inherited gene variants, can tip the balance toward a dangerous clot.
The Three Conditions That Set the Stage
Doctors still organize DVT risk around a framework called Virchow’s Triad, which distills the many risk factors into three basic elements: venous stasis (blood pooling or moving slowly), vascular injury (damage to the inner lining of a vein), and hypercoagulability (a state in which the blood itself is more prone to clotting than normal).1PubMed Central. Deep vein thrombosis: pathogenesis, diagnosis, and medical management Think of these as three dials. When one is turned up, your risk climbs a little. When two or three are turned up at the same time, your risk climbs a lot.
Venous stasis is probably the most intuitive of the three. Blood in your leg veins has to travel upward against gravity to get back to your heart. If you stop moving your legs for an extended period, that blood slows down and pools, especially in the deep veins of the calf. Vascular injury can come from surgery, a fracture, a catheter, or even chronic inflammation irritating the vein wall. And hypercoagulability can be inherited, triggered by hormones, or driven by conditions like cancer. A person lying in a hospital bed after hip surgery, for instance, may have all three dials cranked at once: their legs are immobile (stasis), the surgery damaged nearby vessels (injury), and the body’s inflammatory healing response has ramped up clotting factors (hypercoagulability).
Why Your Calf Muscles Matter More Than You Think
Your leg veins depend heavily on the muscles around them to push blood upward. Every time you flex your calf, the muscles squeeze the deep veins and propel blood toward the heart. This mechanism, sometimes called the calf muscle pump, is a major driver of venous return from the legs. One-way valves inside the veins keep the blood from falling back down between squeezes.
When that pump is weak or inactive, blood pools. A population-based study found that people with reduced calf pump function had roughly double the risk of venous thromboembolism compared to those with normal function.2PubMed Central. Reduced calf muscle pump function is a risk factor for venous thromboembolism: a population-based cohort study That held true even in an otherwise low-risk group of people who were walking around and living normal lives. It helps explain why conditions that weaken calf muscles, from neurological disease to prolonged bed rest to simple deconditioning, raise DVT risk even when nothing else seems wrong.
The anatomy inside the calf also matters. When someone is bedridden, clots tend to form first in the intramuscular veins, particularly the soleal vein, which sits inside the flat muscle at the back of the lower calf. That vein depends almost entirely on muscular squeezing and its own valves for flow, whereas other deep veins in the calf get an assist from the pulsation of nearby arteries.3Annals of Vascular Diseases. Pathophysiology of Venous Thromboembolism with Respect to the Anatomical Features of the Deep Veins of Lower Limbs: A Review So the soleal vein is especially vulnerable when you stop moving.
Immobility, Travel, and the Hypoxia Factor
The classic DVT triggers are situations that keep your legs still for hours: long-haul flights, road trips, post-surgical bed rest, or even sitting at a desk for an entire workday without getting up. In all of these, the calf muscle pump essentially shuts off, and blood pools in the lower legs.
Air travel adds an extra wrinkle beyond just sitting still. At cruising altitude, the cabin is pressurized to the equivalent of roughly 6,000 to 8,000 feet above sea level. That means you are breathing air with less oxygen than at ground level. Research on how this environment affects clotting shows that local hypoxia, the low-oxygen state caused by stagnant blood in the veins, plays a direct role in triggering clot formation. When the overall oxygen level in the cabin drops on top of that, the effect is amplified.4Circulation Research. Cellular and Molecular Mechanisms Leading to Air Travel-Induced Thrombosis So on a long flight you are dealing with both immobility and reduced oxygen, a double hit.
That said, most healthy people who sit through a four-hour flight will not develop a clot. The risk climbs meaningfully with very long flights, existing risk factors like obesity or a clotting disorder, and especially a combination of both. The simplest countermeasure is also the most effective: get up and walk around periodically, flex your calves while seated, and stay hydrated.
Surgery and Injury
Major surgery, especially on the hip or knee, is one of the highest-risk scenarios for DVT. A review of risk factors after joint replacement surgery identified four consistent predictors: a history of prior blood clots, obesity (a body mass index above 25), delayed walking after the procedure, and female sex.5PubMed Central. Risk factors for venous thromboembolism after total hip and knee replacement surgery The surgery itself damages blood vessels near the joint, the patient is immobilized for hours or days afterward, and the body’s wound-healing response temporarily shifts clotting factors upward. All three legs of Virchow’s Triad are engaged simultaneously, which is why surgeons prescribe blood thinners and compression devices as a routine part of post-operative care.
Traumatic injuries to the leg, such as fractures or severe muscle damage, carry similar risk for the same reasons. Even a cast or brace that immobilizes the leg for weeks can be enough to trigger a clot in someone who is otherwise healthy.
Genetic Factors That Make Blood Clot Too Easily
Some people are born with blood that is primed to clot more aggressively than normal. The best-known inherited risk factor is a gene variant called Factor V Leiden. In a study of patients with DVT, carrying Factor V Leiden was associated with roughly a sixfold to sevenfold increase in odds of developing a clot compared to people without the variant.6PubMed Central. Genetic risk factors in patients with deep venous thrombosis, a retrospective case control study on Iranian population Other inherited thrombophilias include the prothrombin G20210A mutation and deficiencies in natural anticoagulant proteins like antithrombin, protein C, and protein S.7PubMed. Genetic risk factors for venous thromboembolism
Carrying one of these variants does not mean you will definitely get a clot. Many people live their entire lives with Factor V Leiden and never develop DVT. The gene variant raises your baseline risk, so when you add a triggering event on top, like surgery, pregnancy, or a long period of immobility, the combination can push you over the threshold. This is why thrombophilia testing is sometimes done after someone has an unexplained DVT, especially if they are young or have a strong family history of clots. Knowing you carry the variant does not change your day-to-day life much, but it changes how aggressively doctors will prevent clots around high-risk events.
Cancer and Clotting
Cancer is one of the strongest acquired risk factors for DVT. Tumors alter the clotting system in several ways at once. Cancer cells produce and release proteins that activate the clotting cascade, as well as inflammatory molecules that shift the blood toward a pro-clotting state. They can also directly interact with platelets, white blood cells, and the cells lining blood vessels, amplifying the effect.8PubMed. Mechanisms and risk factors of thrombosis in cancer On top of that, tumors shed tiny particles called microparticles into the bloodstream that carry clot-promoting material.
Certain cancers are more strongly linked to DVT than others. Pancreatic, brain, lung, and ovarian cancers tend to carry the highest clot risk, though DVT can occur with almost any malignancy. Chemotherapy itself adds risk by damaging blood vessel walls and further disrupting the clotting balance. In fact, an unexplained DVT, one with no obvious trigger like surgery or immobility, is sometimes the first sign that sends doctors looking for a hidden cancer, particularly in older adults.
Obesity, Inflammation, and the Blood
Carrying excess weight raises DVT risk through mechanisms that go beyond the simple idea of more tissue compressing the veins (though that contributes too). Obesity drives a state of chronic, low-grade inflammation throughout the body, and it impairs the fibrinolytic system, the set of enzymes your blood uses to dissolve clots once they start forming. Together, those two effects push the blood toward hypercoagulability.9PubMed Central. Mechanisms of thrombosis in obesity
Fat tissue is not just passive storage. It actively secretes inflammatory signals and hormones that alter how clotting proteins behave in the bloodstream. This is why obesity appears as a risk factor in nearly every DVT study, and why it compounds the risk from other triggers. An obese person who has surgery or takes a long flight faces a higher combined risk than a lean person in the same situation, not just because the veins are under more pressure but because the blood itself is in a more clot-prone state.
Why Risk Rises Sharply with Age
DVT can happen at any age, but the incidence climbs steeply after middle age. Overall estimates put the annual incidence of venous thromboembolism at roughly one to three cases per thousand people, but that number jumps to about eight per thousand in people over 80.10PubMed. Risk factors for venous thromboembolism after total hip and knee replacement surgery Several things converge as you get older. The clotting system becomes more activated at baseline, with higher circulating levels of clotting factors, and the body’s ability to dissolve unwanted clots weakens.11PubMed. Physiological changes due to age. Implications for the prevention and treatment of thrombosis in older patients On top of that, older adults are more likely to have other conditions that raise risk, such as heart failure, cancer, or reduced mobility from arthritis or neurological disease. Age is not something you can modify, but it helps explain why doctors become more aggressive about clot prevention in hospitalized older patients.
When the Anatomy Itself Is the Problem
Sometimes the cause of DVT is not a trigger event or a blood disorder but an anatomical quirk that has been present since birth. The most well-known example is May-Thurner syndrome, in which the right common iliac artery crosses over and compresses the left common iliac vein against the spine.12PubMed Central. May-Thurner syndrome: a not so uncommon cause of a common condition That compression narrows the vein and slows blood flow through it, creating a setup for clot formation on the left side.
May-Thurner syndrome is often described as rare, but some researchers suspect it is underdiagnosed because it may cause no symptoms at all until a clot actually forms. It tends to present in younger adults, often women in their twenties to forties, who develop an unexplained DVT in the left leg.13PubMed Central. Deep Vein Thrombosis of the Left Leg: A Case of May-Thurner Syndrome If you have a DVT in your left leg with no obvious trigger, especially at a young age, this is one of the conditions doctors may investigate. Diagnosis typically involves contrast-enhanced imaging to visualize the compression, and treatment can include stenting the vein open in addition to standard blood thinners.14PubMed Central. May-Thurner syndrome
How DVT Is Diagnosed
DVT symptoms, when they appear, usually include swelling in one leg, pain or tenderness (often starting in the calf), warmth, and redness. But these symptoms overlap with a dozen other conditions, from a pulled muscle to cellulitis to a Baker’s cyst behind the knee. So doctors use a combination of tools rather than relying on symptoms alone.
The standard approach starts with a clinical probability assessment, essentially a checklist of risk factors and symptoms that categorizes you as likely or unlikely to have DVT. If you score low on that checklist and a blood test called D-dimer comes back negative, DVT can be ruled out without imaging. A negative D-dimer in a low-probability patient means the chance of a clot is so small that ultrasound is unnecessary.15PubMed. Evaluation of D-dimer in the diagnosis of suspected deep-vein thrombosis If either the clinical score is high or the D-dimer is elevated, the next step is compression ultrasound, in which a technician presses the ultrasound probe against the vein. A healthy vein collapses easily under pressure; a vein with a clot inside it does not.16PubMed. Noninvasive diagnosis of deep venous thrombosis. McMaster Diagnostic Imaging Practice Guidelines Initiative
One important caveat: D-dimer is a sensitive test but not a specific one. Lots of things besides DVT can raise D-dimer levels, including recent surgery, infection, pregnancy, cancer, and even normal aging. A positive D-dimer does not confirm a clot. It just means you need more testing. This is why the clinical probability assessment matters so much. Without it, you would be sending nearly everyone with a positive D-dimer for an ultrasound, which would be inefficient and anxiety-provoking for patients who never had a clot in the first place.
What Happens If a Clot Is Not Caught
The most feared complication of DVT is pulmonary embolism, or PE, in which part or all of the clot breaks loose, travels through the bloodstream, and lodges in the arteries of the lungs. A large PE can be fatal within minutes. Smaller emboli may cause chest pain, shortness of breath, and a rapid heart rate. DVT and PE are considered two manifestations of the same disease, which is why the umbrella term venous thromboembolism (VTE) covers both.
Even when a DVT does not throw off a dangerous embolus, it can leave lasting damage. Post-thrombotic syndrome, or PTS, develops in a significant number of people after a DVT and results from long-term injury to the vein and its valves.17PubMed Central. Incidence and interventions for post-thrombotic syndrome Symptoms include chronic leg swelling, aching, heaviness, skin discoloration, and in severe cases, open ulcers near the ankle. PTS is essentially a venous stress disorder: the damaged valves can no longer prevent blood from pooling, leading to persistently high pressure in the veins of the affected leg. The severity ranges from mild nuisance to genuinely debilitating, and it tends to be worse when the original clot was large or located high in the thigh.
Prevention Beyond Blood Thinners
For people in high-risk situations, prevention strategies extend beyond anticoagulant medications. Graduated compression stockings, which are tightest at the ankle and gradually loosen up the leg, help push blood upward and reduce pooling. Intermittent pneumatic compression devices, basically inflatable cuffs that rhythmically squeeze the calves, mimic the action of the calf muscle pump when the patient cannot walk. In certain extreme-risk patients who cannot take blood thinners, an inferior vena cava filter, a small metal device placed inside the body’s largest vein, can catch clots before they reach the lungs.
For everyday life, the most effective preventive measures are also the simplest. Regular physical activity keeps the calf muscles strong and the blood moving. On long trips, getting up to walk every hour or two, or at minimum doing seated calf raises, helps maintain venous flow. Staying well hydrated prevents the blood from becoming more concentrated. If you know you carry a genetic risk factor, alerting your doctors before surgery or during pregnancy allows them to calibrate clot prevention accordingly. And if you notice one-sided leg swelling, pain, or warmth that develops over hours to days, getting evaluated promptly rather than waiting it out is one of the most consequential decisions you can make, because early treatment with anticoagulants can prevent the clot from growing, prevent PE, and reduce the likelihood of long-term vein damage.
Hormones, Pregnancy, and Oral Contraceptives
Estrogen has a well-established effect on the clotting system. It increases the production of several clotting factors in the liver and simultaneously decreases some of the body’s natural anticoagulant proteins. This is why pregnancy, which produces high estrogen levels along with physical compression of the pelvic veins by the growing uterus, raises DVT risk several-fold compared to a non-pregnant state. The risk peaks in the weeks immediately after delivery, when the body is still recovering from the hormonal and physical stress of childbirth.
Hormonal birth control, particularly combined estrogen-progestin pills, also raises DVT risk. The absolute risk for any individual woman remains low, but it is measurably higher than for women not taking hormonal contraception. The newer progestin-only methods, such as certain IUDs and implants, appear to carry less clotting risk because they avoid the estrogen component. Hormone replacement therapy in postmenopausal women follows a similar pattern: formulations containing estrogen increase clot risk, and the first year of use tends to carry the highest relative risk before the body somewhat adapts.
For most women, the risk from hormonal contraception is small enough that the benefits outweigh it. But the calculation changes if you also carry a genetic clotting variant like Factor V Leiden or have other risk factors stacked up. A woman with Factor V Leiden who takes combined oral contraceptives faces a multiplicative increase in DVT risk, not just an additive one. This is one of the few situations where knowing your genetic status can directly change a medical decision.
Smoking, Dehydration, and Other Overlooked Contributors
Smoking damages the endothelium, the thin layer of cells lining your blood vessels, and promotes a chronic inflammatory state in the vascular system. Both effects tilt the balance toward clotting. The risk from smoking alone is modest, but it amplifies other risk factors. A smoker who also takes estrogen-containing contraception, for example, faces a higher combined clot risk than either factor would produce on its own.
Dehydration is another commonly underestimated contributor. When you lose fluid through sweating, illness, or simply not drinking enough, the blood becomes more viscous and flows more sluggishly. On its own, mild dehydration is unlikely to cause a clot in a healthy person. But combined with immobility or a flight at altitude, it adds another nudge toward stasis. This is one reason physicians emphasize fluids during and after surgery, during long travel, and during illnesses that cause vomiting or diarrhea.
Certain chronic medical conditions also raise DVT risk in ways that are not always obvious. Heart failure reduces the heart’s ability to efficiently pump blood back from the extremities. Inflammatory bowel disease and autoimmune conditions create systemic inflammation that activates the clotting cascade. Even severe infections like sepsis can trigger widespread clotting. In hospitalized patients, it is often a combination of the underlying illness, the immobility of being in bed, and sometimes an intravenous catheter irritating a vein that creates the perfect storm for DVT formation.