Causes of Blood Clots in Legs: Risk Factors, Symptoms, and How to Assess Your Risk

Every year, up to 900,000 people in the United States develop a venous thromboembolism, according to the Centers for Disease Control and Prevention. Many of them had no idea they were at risk. Understanding the causes of blood clots in legs is one of the most important steps a person can take to protect their long-term health, especially before surgery, hospitalization, or any period of reduced mobility. 

This page breaks down the medical, lifestyle, and situational factors that contribute to clot formation, what the science says about cumulative risk, and how a validated risk assessment tool can help identify your personal risk level before a clot ever develops. 

Understanding Blood Clots in the Legs

A blood clot in the leg is medically referred to as Deep Vein Thrombosis (DVT). It occurs when a clot forms in one of the deep veins, most commonly in the calf, thigh, or pelvis. While the body naturally forms clots to stop bleeding after an injury, a clot that forms inside a vein without an obvious trigger is a serious medical concern.

When a DVT breaks free and travels through the bloodstream to the lungs, it becomes a pulmonary embolism. According to the CDC, PE occurs when part of a DVT clot breaks off and travels to the lungs and can be life-threatening.

Identifying your risk early is the single most effective way to prevent this from happening. The Caprini Risk Score was built on exactly this principle, using a validated, point-based system to quantify individual risk before complications arise.

What Causes DVT?

To understand why blood clots form in the legs, it helps to start with the foundational science. In the 19th century, German physician Rudolf Virchow identified three core conditions that contribute to abnormal clot formation inside blood vessels. These three conditions, now known as Virchow’s Triad, remain the cornerstone of modern DVT science.

Slow or Restricted Blood Flow

When blood moves slowly through the veins, it pools and creates the conditions for clotting. This is why prolonged bed rest, long flights, sitting for extended periods, or immobilization after surgery all increase clot risk significantly. The calf muscles act as a natural pump for venous blood return. When those muscles are inactive, that pump stops working.

Damage to the Vein Wall

Injury to the inner lining of a vein, whether from surgery, trauma, inflammation, or a catheter insertion, activates the body’s clotting response. This is a normal part of wound healing, but in certain conditions it can trigger clot formation inside a vein where it is not needed and cannot safely resolve on its own.

Blood That Clots Too Easily

Some people have blood that is hypercoagulable, meaning it has a heightened tendency to clot. This can be the result of inherited genetic mutations, certain medications, cancer, pregnancy, or chronic inflammatory conditions. In these cases, even normal blood flow and healthy vein walls may not be enough to prevent clot formation.

Understanding which of these three mechanisms applies to your situation is essential. In most real-world cases, more than one is present at the same time. You can learn more about how these mechanisms are factored into clinical risk assessment on our Caprini Risk Score page.

Medical Conditions That Increase Your Risk

Several underlying health conditions directly contribute to the causes of blood clots in legs by affecting blood composition, vein health, or circulation. Recognizing these conditions as risk factors is critical for anyone preparing for surgery or a hospital stay.

Inherited Clotting Disorders

Genetic mutations such as Factor V Leiden significantly increase the blood’s tendency to clot. According to Cleveland Clinic, Factor V Leiden affects approximately 5 in 100 people of European descent in the US, and around 1 in 5 people who develop a blood clot in their veins carry this variant. People with this disorder may experience clots with little or no provocation.

Chronic Illness and Cardiovascular Disease

Cancer is one of the strongest known triggers for DVT. According to the CDC, cancer patients receiving chemotherapy have a much higher risk of DVT than other people, and pulmonary embolism is the leading cause of death in cancer patients after the cancer itself. Patients with brain, pancreatic, stomach, and lung cancers face the highest individual risk.

Inflammatory conditions including Crohn’s disease, ulcerative colitis, and congestive heart failure also create an internal environment where clotting is more likely. Patients with a history of stroke, varicose veins, or prior DVT carry a significantly elevated baseline risk that must be factored into any pre-surgical assessment. Our resources section on venous disease covers many of these conditions in greater detail.

Lifestyle and Situational Risk Factors

Not all blood clot risk factors are medical in nature. Everyday habits and situational circumstances play a significant role in whether a clot develops, particularly in combination with other underlying vulnerabilities.

Prolonged Immobility and Travel

Sitting still for four hours or more, whether on a long-haul flight, a road trip, or at a desk, reduces blood flow through the legs dramatically. According to the WHO WRIGHT project, the risk of developing VTE approximately doubles after travel lasting four hours or more, whether by plane, train, bus, or car. This risk is further compounded when additional factors such as obesity or hormonal contraceptive use are present. 

Obesity and Smoking

Obesity increases abdominal pressure and impairs venous return from the lower limbs. A meta-analysis found that obese individuals face more than twice the risk of DVT compared to those with a healthy body weight. Smoking damages the vein walls and promotes a chronic inflammatory state that accelerates clot formation, compounding risk further when combined with other factors. 

Key lifestyle blood clot risk factors at a glance:

  • Sitting or lying still for extended periods 
  • Long-distance air or road travel without movement breaks 
  • Obesity (BMI over 30) 
  • Active smoking or recent cessation 
  • Dehydration during travel or illness 
  • Sedentary occupation with limited daily movement 

Surgery, Hospitalization and Blood Clot Risk

The period immediately following surgery is one of the highest-risk windows for DVT development. General anesthesia slows circulation, post-operative immobility removes the calf muscle pump effect, and surgical trauma activates the body’s clotting response simultaneously.

The causes of blood clots in legs are especially concentrated in this context. The American College of Chest Physicians assigns its highest level of evidence, Grade 1A, to the recommendation that all patients undergoing hip or knee arthroplasty receive anticoagulant thromboprophylaxis, reflecting the serious and well-documented VTE risk these procedures carry. This makes orthopedic procedures among the highest-risk surgical events for VTE.

This is precisely the clinical environment for which the Caprini Risk Score was developed. By quantifying a patient’s individual risk profile before surgery, clinicians can implement targeted prophylaxis at exactly the right level for each patient. The score has been validated in nearly 5 million patients across more than 200 peer-reviewed publications.

Hormones, Age and Other Contributing Factors

Beyond medical conditions and lifestyle, certain biological and demographic factors contribute meaningfully to clot risk. These are not always within a person’s control, but understanding them allows for better preparation and monitoring.

Pregnancy and Postpartum Risk

Pregnancy increases clot risk through multiple pathways. According to the Royal College of Obstetricians and Gynaecologists, the risk of VTE is four to five times higher in pregnant women than in non-pregnant women of the same age, and the postpartum period carries the highest risk window of all, with a relative risk of approximately 20-fold compared to baseline. 

Age as a Risk Factor

Age is one of the most consistent and well-documented blood clot risk factors in the medical literature. The risk of DVT roughly doubles with each decade of life after age 40. This is partly due to reduced mobility, higher rates of chronic illness, and age-related changes in blood composition. The Caprini Risk Score assigns specific point values to age brackets for exactly this reason.

Estrogen-based oral contraceptives and hormone replacement therapy also elevate clot risk, particularly in women over 35 who smoke. Patients taking these medications should discuss their VTE risk profile with their physician before any surgical procedure.

How Multiple Risk Factors Work Together

One of the most important and least understood aspects of DVT is that risk is cumulative. A single risk factor in isolation may present a moderate threat. Two or three factors present simultaneously can elevate that threat dramatically. 

Consider a patient who is 55 years old, recently underwent abdominal surgery, and is spending several days at bed rest during recovery. Each of those three factors adds to the total risk. Combined, they create a high-risk profile that requires active prophylaxis. 

This cumulative principle is the foundation of the Caprini Risk Score. Rather than treating each risk factor independently, the scoring model assigns weighted point values to individual factors and combines them into a single total score. That score then maps to a defined risk category, from low to highest risk, which guides clinical decision-making on prophylaxis. Understanding the causes of blood clots in legs is not just about identifying one trigger. It is about recognizing how your complete health picture adds up. 

Early Warning Signs You Should Not Ignore

Recognizing the symptoms of a DVT early can prevent a clot from becoming life-threatening. Many people dismiss the early signs of a blood clot as muscle soreness or minor swelling, which is why DVT is frequently underdiagnosed until it progresses. 

Common warning signs of a blood clot in the leg include: 

  • Swelling in one leg, often in the calf or thigh 
  • A dull ache, cramping, or soreness that does not improve with rest 
  • Skin that feels warm or hot to the touch around the affected area 
  • Redness or a bluish discoloration of the skin 
  • A feeling of heaviness or tightness in the leg 

If a clot dislodges and travels to the lungs, symptoms escalate rapidly. Sudden shortness of breath, chest pain, rapid heartbeat, or coughing up blood are signs of a pulmonary embolism and require immediate emergency care. Do not wait to see if symptoms resolve. Call emergency services immediately. You can also learn more about additional risk factors that can worsen these outcomes on our dedicated resource page. 

How Doctors Diagnose and Assess Blood Clot Risk

When DVT is suspected, physicians use a combination of imaging and laboratory tests to confirm the diagnosis. The most widely used diagnostic tool is duplex ultrasound, a non-invasive imaging technique that can detect clots in the deep veins of the leg with high accuracy. A D-dimer blood test is often used alongside ultrasound to measure clot-related proteins in the bloodstream. Elevated D-dimer levels indicate active clotting activity, though the test alone is not sufficient for a definitive diagnosis. 

Beyond diagnosis, proactive assessment of blood clot risk factors is increasingly recognized as an essential part of patient care. Validated tools that evaluate individual risk before symptoms appear allow clinicians to intervene before a clot forms rather than after. 

The Caprini Risk Score is the most widely validated preoperative risk assessment tool of its kind, used in hospitals across North America, Europe, Asia, and the Middle East. By evaluating over 40 individual risk factors and assigning weighted scores, the tool produces a risk category that guides prophylaxis decisions with a level of precision that general clinical judgment alone cannot match. Read more about how the score is used in national clinical guidelines. 

Know Your Causes of Blood Clots in the Legs Before It Is Too Late

The causes of blood clots in legs span genetics, lifestyle, medical history, age, and circumstance. No single factor tells the whole story, and no two patients carry the same risk profile. That is exactly why individualized risk assessment matters. 

The Caprini Risk Score gives patients and clinicians a clear, validated, evidence-based picture of where that risk stands. Whether you are preparing for surgery, managing a chronic condition, or simply want to understand your personal risk level, taking the assessment is the most informed first step you can take. 

Take the Caprini Risk Score Assessment today and know your risk before it becomes a crisis. 

  • Contact the team with any questions about your score or what it means for your care 

Frequently asked questions about travel-related thrombosis

The most common causes include prolonged immobility, recent surgery, inherited clotting disorders, obesity, cancer, hormone therapy, and advanced age. In most cases, more than one factor is present at the same time, and the combination of factors determines overall risk level. You can assess your personal risk using the Caprini Risk Score assessment. 

Chronic stress can contribute indirectly to clot risk by elevating cortisol levels, promoting inflammation, and increasing blood viscosity. Emerging research suggests a measurable link between psychological stress and elevated VTE risk, though stress is rarely the sole cause and should be considered alongside other established risk factors. 

Surgery activates the body’s clotting response through tissue trauma, reduces mobility during recovery, and often involves general anesthesia that slows circulation. These three mechanisms align directly with Virchow’s Triad, making the post-surgical period one of the highest-risk windows for DVT development. Learn more about how the Caprini Score is used in surgical settings. 

Virchow’s Triad refers to the three conditions that contribute to abnormal clot formation: slow or restricted blood flow, damage to the vein wall, and blood that clots too easily. Identified by Rudolf Virchow in the 19th century, this framework remains the foundation of modern DVT science and underpins the risk factor categories evaluated by the Caprini Risk Score.

The Caprini Risk Score evaluates over 40 individual risk factors and assigns each a weighted point value. The total score places a patient into a defined risk category that guides clinical decisions on prophylaxis. It has been validated in nearly 5 million patients and is used in hospitals across the globe to prevent deaths from preventable blood clots.

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