What is deep vein thrombosis?

Deep vein thrombosis (DVT) is a serious medical condition that occurs when a blood clot forms in a deep vein, most commonly in the legs. These clots can block normal blood flow, causing pain and swelling. Even more dangerous, a clot can break loose, travel to the lungs, and cause a life-threatening complication known as pulmonary embolism (PE).

DVT can affect anyone, but certain factors make some people much more susceptible than others. Understanding what causes DVT and identifying risk factors early can prevent long-term complications or even save a life.

How blood clots form

Blood clots are a normal part of the body’s healing process. When you cut yourself, your body forms a clot to stop bleeding. But sometimes, clots form where they shouldn’t — inside a vein without injury. This usually happens due to one or more components of Virchow’s triad:

  1. Venous stasis (slow or stagnant blood flow)

  2. Hypercoagulability (increased tendency of blood to clot)

  3. Endothelial injury (damage to the vessel lining)

Most DVT risk factors fall into one or more of these three categories.

Prolonged immobility

When you stay in one position for a long time, blood flow slows down in the lower limbs. This can lead to clot formation, especially in the deep veins of the legs.

Common situations that cause immobility:

  • Long-distance flights or car rides (over 4 hours)

  • Bed rest during illness or hospitalization

  • Sedentary lifestyle or desk jobs without breaks

  • Post-surgery recovery

Standing or walking at regular intervals helps maintain healthy circulation and lowers your DVT risk.

Surgery and trauma

Surgical procedures, especially orthopedic surgeries like hip or knee replacements, significantly increase DVT risk. This is due to:

  • Tissue trauma triggering clotting

  • Long periods of immobility post-surgery

  • Inflammation and vessel damage

Major trauma (such as fractures or car accidents) can also damage blood vessels, prompting abnormal clotting. Doctors usually prescribe blood thinners and compression devices during recovery to lower the risk.

Cancer and cancer treatments

Cancer increases clotting risk due to the body’s inflammatory response and substances secreted by tumors that activate clotting pathways.

Cancers most strongly associated with DVT include:

  • Pancreatic cancer

  • Lung cancer

  • Brain tumors

  • Gastrointestinal cancers

Additionally, chemotherapy and certain cancer surgeries contribute to higher risk. Some patients may need long-term anticoagulation to manage this risk.

Pregnancy and postpartum

Pregnancy naturally increases the blood’s ability to clot — a protective mechanism to prevent excessive bleeding during childbirth. But this also raises the chance of DVT.

DVT risk is highest:

  • In the third trimester

  • After delivery (especially cesarean section)

  • In women with preeclampsia or bed rest orders

Hormonal changes, reduced venous return from the growing uterus, and less mobility all contribute. Postpartum women should be monitored closely and encouraged to walk as soon as safely possible.

Hormonal medications

Estrogen-containing medications can increase clotting risk by affecting the coagulation cascade.

Medications that increase DVT risk:

  • Oral contraceptives (birth control pills)

  • Hormone replacement therapy (HRT)

  • Fertility treatments

The risk is even higher in women who smoke, are over 35, or have other DVT risk factors. Low-estrogen or non-hormonal alternatives may be safer in these cases.

Genetic clotting disorders

Inherited conditions can make a person’s blood more likely to clot — a state called thrombophilia.

Common inherited disorders include:

  • Factor V Leiden mutation

  • Prothrombin gene mutation

  • Protein C or S deficiency

  • Antithrombin III deficiency

Many people with these conditions don’t experience symptoms until they develop a clot. Testing is usually done after an unexplained or recurrent DVT, especially at a young age or with a family history.

Obesity

Being overweight increases the pressure in the veins of the pelvis and legs, slowing blood flow. Obesity is also associated with systemic inflammation and hormonal imbalances that can contribute to hypercoagulability.

Key risks of obesity and DVT:

  • Reduced mobility

  • Elevated estrogen levels

  • Poor circulation in the lower body

Even moderate weight loss can significantly reduce DVT risk and improve overall vascular health.

Smoking

Smoking damages the lining of blood vessels (endothelium), triggers inflammation, and increases clotting factor levels. It’s a well-established and preventable risk factor for both DVT and arterial clots.

Smoking combined with other risk factors (like birth control or surgery) multiplies the danger. Quitting smoking not only protects your lungs and heart but also reduces your risk of potentially fatal blood clots.

Age and gender

DVT can happen at any age, but the risk increases significantly after age 60 due to slower circulation and greater incidence of chronic conditions. However, young people are not immune — especially if other risk factors are present.

In terms of gender:

  • Women have a slightly higher risk during reproductive years (due to hormones, pregnancy, and contraceptive use)

  • Men are more likely to experience recurrent DVT

Age-related screenings and preventive strategies become more important with advancing years.

Chronic illnesses

Certain long-term conditions can elevate the risk of DVT, including:

  • Heart failure

  • Chronic obstructive pulmonary disease (COPD)

  • Inflammatory bowel disease (Crohn’s and ulcerative colitis)

  • Kidney disease

  • Autoimmune conditions (e.g., lupus)

These illnesses can contribute to inflammation, immobility, and vascular damage — creating the perfect environment for clot formation.

Dehydration

Lack of adequate fluid intake can thicken the blood, especially in hot weather or during physical exertion. This increases clotting potential, particularly when combined with immobility or travel.

Stay hydrated by:

  • Drinking water regularly throughout the day

  • Avoiding excessive alcohol or caffeine on long trips

  • Monitoring hydration during illness, fever, or exercise

Good hydration is a simple yet powerful tool for reducing DVT risk.

Varicose veins and venous insufficiency

While not a direct cause, varicose veins and chronic venous insufficiency can slow blood return and increase the likelihood of clots forming in the legs.

Symptoms to monitor:

  • Leg heaviness

  • Swelling

  • Skin discoloration

  • Visible, twisted veins

Compression stockings and leg elevation can help reduce risk in people with vein disorders.

Inflammation and infection

Inflammation from infections or inflammatory diseases increases clotting factor production. Infections like pneumonia, COVID-19, and sepsis have been linked to higher rates of DVT and PE.

Even temporary inflammation can tip the balance toward clot formation, especially in hospitalized or immobilized patients. Proactive care during and after illness is essential.

Travel and flying

Long-distance travel (over 4 hours), especially by air, increases the risk of DVT due to prolonged sitting and reduced leg movement. This risk is higher if combined with other factors like obesity, recent surgery, or a history of clots.

Preventive tips for travel:

  • Stand and stretch every 1–2 hours

  • Wear compression stockings

  • Stay hydrated

  • Avoid tight clothing around the legs

  • Consider low-dose aspirin or anticoagulants if prescribed by a doctor

Awareness and simple actions can dramatically lower your travel-related risk.

How to reduce your risk

Whether you have one risk factor or several, there are many proactive ways to protect yourself:

  • Move frequently, especially during travel or work

  • Exercise regularly to improve circulation

  • Stay hydrated

  • Lose excess weight

  • Stop smoking

  • Wear compression stockings if recommended

  • Manage chronic conditions with regular checkups

  • Take prescribed medications (anticoagulants) after surgery or during hospital stays

If you’re unsure of your risk, talk to your doctor about testing or screening options.

Recognizing DVT symptoms

Early detection saves lives. Common signs of DVT in the legs include:

  • Swelling (usually in one leg)

  • Pain or tenderness

  • Warmth over the vein

  • Red or discolored skin

If you experience sudden chest pain, shortness of breath, or coughing up blood — it could indicate a pulmonary embolism and requires emergency care.

Conclusion

Deep vein thrombosis is a serious but preventable condition. From immobility and genetics to hormonal therapy and chronic illness, DVT arises from a wide range of causes. Fortunately, with the right awareness, lifestyle adjustments, and medical guidance, you can take meaningful steps to reduce your risk.

Understanding what causes DVT and acting early could protect you from life-threatening complications and ensure better vascular health for years to come.