Top causes of blood clots in the legs and how to avoid them

Don’t let a plane ride or an extended couch potato session put you at risk. Here’s what to do to keep your blood flowing.

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?

Causes and triggers

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

• being bedridden for long periods because of surgery or illness
• sitting for long periods — even three to four hours — in a car, plane, or train
• getting too little activity and sitting too much
• having blood pool in your legs because valves in a superficial vein don’t work properly (a varicose vein)
• taking a medication that promotes blood clotting.

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.

Symptoms and risks

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.

Avoiding blood clots

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.

What if you have symptoms?

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."