Blood clot risk factors: what raises the risk of DVT and PE

DVT, PE and VTE in plain words

Venous thromboembolism (VTE) is a blood clot that forms in a vein. It includes deep vein thrombosis (DVT), a clot in a deep vein, usually in the lower leg, thigh or pelvis, and pulmonary embolism (PE), when a clot breaks loose and travels through the bloodstream to the lungs. VTE is common: as many as 600,000 VTE events occur each year in the United States (NHLBI). DVT is the most common type of VTE (NHLBI DVT). If a clot is large or there are many clots, a PE can cause death (NHLBI).

Risk factors that raise the risk

Most often more than one risk factor is involved, and the more risk factors you have, the greater the chance of VTE (NHLBI causes). The risk is highest after major surgery, major injury, or during periods of infection and inflammation (NHLBI).

• Surgery, injury and a hospital stay. Clots can form in veins damaged by surgery or injury. The chance of a clot is highest in the first 3 months after surgery and lowers with time (NHLBI causes). MedlinePlus names joint replacement surgery and a broken hip or leg bone among the things that raise the risk of PE (MedlinePlus PE).
• Not moving for long periods, such as bed rest in a hospital or nursing home, after surgery, or a cast (NHLBI causes) (MedlinePlus PE).
• Long trips. Long flights (more than 4 hours) more than double the likelihood of VTE (NHLBI prevention). MedlinePlus also names long car rides (MedlinePlus).
• A past clot or a clotting disorder (NHLBI causes).
• Cancer and cancer treatments, including chemotherapy and surgery (NHLBI causes).
• Pregnancy and giving birth. Women have a higher chance of VTE during the first 6 weeks after giving birth, and the likelihood is also higher than usual during pregnancy (NHLBI women). MedlinePlus says the risk of PE is highest for about six weeks after childbirth (MedlinePlus PE). Pregnancy can make you more than 5 times more likely to have a blood clot during pregnancy or right after delivery than someone who is not pregnant. Bed rest in pregnancy and a cesarean delivery can raise the chance too (NHLBI women).
• Estrogen and other hormones. Some types of birth control pills and hormone therapy raise the likelihood of clots, and the chance tends to be highest in the first few months after starting them (NHLBI women).
• Age. VTE can occur at any age, but the chance nearly doubles every 10 years after age 40 (NHLBI causes).
• Obesity, which can lead to inflammation and damage in the lining of blood vessels (NHLBI causes).
• Family history. Researchers have found dozens of gene changes that can raise the chance of VTE. The likelihood among brothers and sisters of someone who has had VTE is more than double that of the general population (NHLBI causes).
• Other medical conditions: diabetes, heart attack or heart failure, high blood pressure, infections such as COVID-19, kidney disease, spinal cord injury (NHLBI causes), lupus (NHLBI DVT), atrial fibrillation (MedlinePlus) and lung diseases (MedlinePlus PE).
• Smoking (MedlinePlus).
• Sex. Before menopause, women are more likely to get VTE than men; after menopause, the likelihood is higher in men (NHLBI causes).

Warning signs

Symptoms of PE can come on within seconds to minutes, or more slowly over days to weeks (NHLBI PE). Sometimes VTE causes no obvious signs at all (NHLBI), and a DVT can lead to a life-threatening PE (NHLBI DVT).

What lowers the risk

If you are going to the hospital for a procedure or have other VTE risk factors, the NHLBI advises talking with your provider about a plan to prevent clots (NHLBI prevention).

• After surgery or illness: move around as soon as you can; if you cannot walk, flex and stretch your feet. Your provider may suggest compression stockings or a sleeve or boot that fills with air, and may prescribe a blood thinner (NHLBI prevention).
• On long trips: stand up and walk around every 1 to 2 hours if possible, flex your knees and ankles often, change position and avoid crossing your legs, wear loose clothing and compression stockings, and avoid alcohol (NHLBI prevention).
• Every day: regular physical activity, not smoking and staying at a healthy weight (MedlinePlus).
• If you have a high risk of clotting and want to prevent pregnancy: ask about birth control without estrogen, such as an IUD, progestin-only pills or implants, or barrier methods (NHLBI women).

Tools that score clot risk

These scores are used by clinicians. They help organize the facts, but they do not diagnose a clot or replace a visit.

Common questions

Can a long flight really cause a clot?

It can raise the chance. Long flights (more than 4 hours) more than double the likelihood of VTE, especially along with other risk factors (NHLBI prevention) (NHLBI causes). Getting up to walk every 1 to 2 hours and moving your ankles helps (NHLBI prevention).

My parent had a clot. What should I do?

Tell your provider. Some inherited gene changes make blood more likely to clot (NHLBI causes), and your provider can weigh this before surgery, pregnancy or starting hormones.

I am pregnant and take a blood thinner for a past clot. Is that safe?

Talk to your provider, since some blood-thinning medicines are not safe to take during pregnancy (NHLBI women). Do not stop or change a medicine on your own.