Patients often ask me, "Why did I get a blood clot?" It is a good question, because the answer helps prevent the next one. The causes of DVT usually involve more than one thing at once. For example, a long car ride after surgery, or a new hormone pill in someone with a family history of clots. In this article, I explain how deep vein thrombosis (DVT) forms, which risk factors matter most, and what you can do to lower your risk.
Key Takeaways
- Most causes of DVT fall into three groups: slow blood flow, vein injury, and blood that clots too easily.
- Clots usually happen when two or more risk factors overlap, such as surgery plus limited movement.
- Major risk factors include recent surgery, long travel, cancer, pregnancy, estrogen, and a personal or family history of clots.
- Moving often, staying hydrated, and planning ahead before surgery or long trips can lower your risk.
- Sudden chest pain or shortness of breath can mean a clot reached the lungs. Call 911 right away.
What Is Deep Vein Thrombosis?
DVT is a blood clot in a deep vein, most often in the calf, thigh, or pelvis. It can cause swelling, pain, warmth, and redness in one leg. However, some clots cause no symptoms at all.
The biggest danger is a pulmonary embolism (PE). This happens when part of the clot breaks off and travels to the lungs. Over time, DVT can also damage the vein valves. As a result, some people develop long-term leg swelling, skin changes, and even ulcers. Doctors call this post-thrombotic syndrome.
The Three Main Causes of DVT
Doctors describe three forces that lead to a clot. Together, they are called Virchow's triad. Most people who develop DVT have more than one of them at play.
1. Slow blood flow (stasis). Your calf muscles act like a pump. Each time they squeeze, they push blood back toward the heart. So when you sit or lie still for hours, blood moves more slowly and is more likely to clot. This happens with long flights or car rides, bed rest, leg casts, paralysis, or after a stroke. Heart failure and dehydration can also slow blood return.
2. Injury or pressure on the vein. Anything that damages or squeezes a vein can trigger a clot. Common examples include surgery, broken bones, and other injuries. Catheters and devices inside a vein, such as PICC lines, chemo ports, dialysis catheters, and pacemaker wires, can also irritate the vein. In fact, these devices are a leading cause of clots in the arm.
3. Blood that clots too easily. Some people inherit a tendency to clot. Others develop it because of an illness, a medicine, or pregnancy. Often, they do not know about it until a clot forms.
Common Risk Factors for DVT
The National Heart, Lung, and Blood Institute lists many risk factors for blood clots. In my practice, these are the ones I see most often.
- Recent surgery or hospital stay, especially hip, knee, or major cancer surgery. Risk stays higher for several weeks afterward.
- Long travel. Sitting still on a flight or drive of more than about four hours raises risk. Read my tips on DVT on long flights.
- Personal or family history of clots. Having one clot raises your risk of another.
- Cancer and some cancer treatments, including certain chemotherapy drugs.
- Pregnancy and the weeks after delivery. Hormones and pressure on pelvic veins both play a part.
- Estrogen, found in many birth control pills and in hormone therapy for menopause.
- Age. Risk rises as we get older, especially after 60. Still, younger people can get DVT too.
- Obesity, which adds pressure on the veins in the pelvis and legs.
- Smoking, which damages blood vessels and makes blood more likely to clot.
- Long-term illness, such as heart failure, kidney disease, inflammatory bowel disease, and lupus.
Less Common Causes Worth Knowing
Sometimes, the cause is hidden in a person's anatomy or blood chemistry. For instance, in May-Thurner syndrome, an artery presses on the left iliac vein in the pelvis. This can lead to clots in the left leg, often in younger adults. Similarly, repeated overhead arm activity can squeeze a vein near the collarbone and cause a clot in the arm.
Inherited clotting conditions include factor V Leiden and the prothrombin gene mutation. Acquired conditions include antiphospholipid syndrome, an autoimmune disorder. These do not guarantee a clot. However, they raise the risk, especially when combined with surgery or immobility.
Why Most Clots Have More Than One Cause
DVT is rarely caused by a single factor. Instead, risks stack on top of each other. A person might have a mild inherited tendency and then take a long flight. Or a patient might start estrogen therapy shortly before a knee surgery. Together, these tip the balance toward clotting.
That is why I take a full history. I ask about recent travel, surgery, medicines, supplements, family clotting history, and any devices like PICC lines. If you want to know what to watch for, see my guide to the signs of DVT.
How to Lower Your Risk
You cannot change your age or your genes. Fortunately, many other risk factors are within your control.
- Move often. On long trips, stand up or do calf raises every hour or so.
- Drink plenty of water, especially when you travel.
- Keep a healthy weight and stay active with regular walking.
- Quit smoking.
- Talk with your doctor about the clot risk of any hormone medicine.
- Before surgery, ask about a prevention plan, such as early walking, compression, or a blood thinner.
- If you have had a clot before, tell your care team before future procedures or long trips.
Your doctor may also suggest compression stockings in some situations.
When to Get Help
New swelling, pain, or warmth in one leg should be checked promptly, especially after surgery, travel, or illness. In my office, a duplex ultrasound can confirm or rule out a clot quickly and without radiation. If a clot is found, treatment usually starts with blood thinners. You can learn more on our venous thrombosis page.
If you have sudden chest pain, shortness of breath, a racing heart, or you cough up blood, call 911. These can be signs of a pulmonary embolism, which is a medical emergency.
Coastal Vascular Center is a small, physician-owned practice, not a corporate chain. You will see me at every visit, and I personally perform every procedure, with no hand-off to a physician assistant or nurse practitioner. Learn more about Dr. Ayar.
Talk About Your DVT Risk With Dr. Ayar
Have you had a clot before, or do you have new swelling or pain in one leg? We can review your risk factors, check your veins with ultrasound, and build a plan. You will see Dr. Ayar at every visit in Pearland or Lake Jackson.
