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Blood clots that form in the deep veins of the legs (deep-vein thrombosis, or DVT) can cause leg symptoms. Worse, the clots can break loose and travel through the blood to the heart and then to the lungs, causing a pulmonary embolism (PE).

More people die from PE each year in the United States than from breast cancer. What causes DVT and PE, what are the symptoms, and how can you prevent them?

After your arteries bring oxygen-rich blood to your legs, your veins send the blood back up to your heart and lungs (for more oxygen).

"If blood in the deep leg veins doesn’t move fast enough, or if you have a condition that makes you prone to blood clots, a blood clot can develop," explains Dr. Sherry Scovell, a vascular surgeon who specializes in venous disease at Harvard-affiliated Massachusetts General Hospital.

Common blood clot triggers include

Your risk for blood clots also increases with older age, a family history of DVT, a previous DVT, cancer, certain genes, COVID-19, heart failure, obesity, pregnancy, sickle cell disease, smoking, spinal cord injury, stroke, untreated varicose veins, and use of birth control pills or hormone replacement therapy.

Be on the lookout for symptoms of two types of blood clots that can form in the legs.

A blood clot in the superficial veins. This is called a superficial venous thrombosis (SVT). "It causes redness, tenderness, or pain over varicose veins," Dr. Scovell says. "Sometimes, an SVT can grow and become a deep-vein thrombosis."

A blood clot in the deep leg veins. A DVT usually begins in one leg. "When you get a blockage, the blood can’t leave your leg easily. That leg can become swollen rather suddenly, and painful," Dr. Scovell says. If your legs don’t normally get swollen, but one leg becomes swollen over a few days, that may be a sign of danger.

If part of that deep-vein clot breaks off and travels to the lungs, a PE occurs. "The clot gets stuck in blood vessels in the lung, you stop getting enough blood flow there, and that part of the lung dies. You have shortness of breath and chest pain when you take in a deep breath," Dr. Scovell says.

There are apps available to help you determine your risk for getting a DVT. Dr. Scovell recommends an app called "Caprini DVT Risk," available on iOS devices, such as an iPhone.

How can you avoid getting a clot when you’re stuck in situations that increase your risk, such as a long car ride? Keep the following tips in mind.

Stay hydrated. Avoid excessive alcohol intake, and drink lots of water.

Stretch your legs. Get up every hour or two and stretch your calves or move your ankles back and forth repeatedly. "The calf muscles act like pumps and propel blood through the veins," Dr. Scovell says.

Move your legs while you’re lying down. Bend your knees, or point and flex your feet.

Wear compression stockings. They’ll help prevent swelling and keep blood from pooling in the legs.

Pay attention to your position. Avoid crossing your legs, and periodically change your position while seated.

Get an aisle seat when traveling. On a plane, train, or bus, sit in an aisle seat so you can easily get up and move around every few hours.

If you have new symptoms indicating the possibility of a DVT or a PE, and if you can’t speak immediately to your doctor or nurse, go to the emergency room. "It’s an emergency, not something to check out on Monday if it’s Friday," Dr. Scovell says.

Treatment typically involves taking a blood thinner for several months or longer. "We also have to figure out why you got the blood clot. If we can’t find a reason, you may need to take a blood thinner for a longer time," Dr. Scovell says. "And we don’t want you to ever get a blood clot again, so you’ll need to be proactive about avoiding future risks."

As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date of last review or update on all articles.

No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.

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The conditions that contribute to leg pain can range from fractures, muscle tears, and sprains, to long-term problems caused by osteoarthritis, peripheral neuropathy, and vascular disease. This Special Health Report is designed to help you resolve both the pain and the problem. Healing Leg Pain will take you from symptoms to source to solution. You’ll find how to accelerate treatment, avoid recurrence, and assure renewed leg strength and wellness.     

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