Causes and Risk Factors for Blood Clots

Table of Contents
View All
Table of Contents

Blood clots have a wide variety of causes and risk factors. Among them are chronic health conditions, such as diabetes and atrial fibrillation; medications, including birth control pills and hormone replacement therapy; lifestyle factors, such as smoking and being overweight; and, in rare cases, inherited clotting disorders. 


Common Causes

It is important to know your risk factors and talk with your healthcare provider about what, if anything, you can do to lower your risk of a dangerous clot. Blood clots primarily occur in the veins and arteries, interrupting blood flow and potentially leading to heart attacks and strokes.

blood clot causes and risk factors
Illustration by Verywell


When fatty deposits, called plaques, develop in the linings of the arteries (often due to high cholesterol), it is known as atherosclerosis. If a plaque ruptures in a coronary artery, it will cause a blood clot to form, potentially causing permanent damage to the heart muscle, or, worse, a heart attack. 

Atrial Fibrillation

Atrial Fibrillation (AFib) is the most common form of heart arrhythmia, causing your heart to beat too quickly or skip beats, interrupting blood flow. When this happens, blood can pool in the heart and form clots, which can ultimately travel to the brain and lead to a stroke.


Diabetes produces changes in your blood that make it more prone to clotting.

According to the American Heart Association, as many as 80% of people with diabetes are at risk of dying from a clot-related cause.

Prolonged Immobility

Sitting or lying down for long periods—due to prolonged bed rest after illness or a long airplane flight, for example—can cause blood to pool in the legs, leading to deep vein thrombosis (DVT) and, worst-case scenario, pulmonary embolism if the clot travels to the lungs.

Getting up, walking around, and stretching can help you avoid DVT.


A blood clot is more likely to form during or after surgery; one reason for this is prolonged periods of inactivity due to lying on an operating table and staying in bed while recovering. 

The type of surgery you have performed can also increase the risk of blood clots after the procedure. Clots are more common after major surgeries, particularly those involving the pelvis, abdomen, knee and hip.

If surgery requires your arteries or veins to be cut or repaired, the risk of a blood clot is higher because your body works to stop bleeding by forming clots.

A surgery where your heart is stopped, typically a heart bypass surgery (CABG), also ups this risk.

Cancer and Cancer Treatments

Cancer itself, as well as certain chemotherapy drugs, can increase the blood's clotting ability. Cancer patients are also likely to have long periods of inactivity, such as during chemotherapy treatments or while on bed rest.

If you are undergoing treatment for cancer, it's important to be aware of the symptoms of a blood clot. 

Blood Clots Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Old Man


Though they are relatively rare, there are some inherited blood clotting disorders that can make you more prone to developing clots compared to the general population.

Genetic disorders rarely cause blood clots in the arteries. Instead, they are likely to result in deep vein thrombosis (DVT), pulmonary embolism, and clots in the intestines and kidneys.

Factor V Leiden: In factor V Leiden, a substance known as factor V, which is important to the clotting process, can get out of control, causing a benign clot to become dangerous. Between 3 percent and 8 percent of people with European ancestry carry the gene mutation associated with the disorder.

Prothrombin gene mutation: Patients with this disorder have a genetic defect that results in an overabundance of prothrombin, a blood clotting protein. About 2 percent of whites in the United States and Europe have a form of this mutation.

Antithrombin, protein C, and protein S deficiencies: Patients with these rare mutations have a reduced amount of natural anticoagulants in their blood and are thus more prone to clotting.

You’re more likely to have a genetic cause of excessive blood clotting if you have family members who have had dangerous blood clots, a personal history of repeated blood clots before the age of 40, and/or a personal history of unexplained miscarriages.

Lifestyle Risk Factors

While genetic disorders and certain chronic conditions are not things you can control, the following lifestyle-associated risk factors generally are. Your healthcare provider can help you find ways to modify your behaviors and choices to lower your risk of a blood clot. 


Over time, smoking can damage the lining of the blood vessels, making clots more likely. If you have another risk factor, such as being pregnant or using birth contorl, your risk is further increased.

Talk to your healthcare provider about a smoking cessation program if you need help quitting cigarettes. 


Carrying extra fat can slow blood flow and put more pressure on your veins. Being significantly overweight can sometimes coincide with an inactive lifestyle and/or diabetes, both risk factors in and of themselves.

A nutritionist or a group weight-loss program can help you learn how to make healthy food choices and start an exercise program. 

Pregnancy and Postpartum

Pregnancy increases the number of platelets and clotting factors in the blood, increasing a woman's chance of developing a clot. The uterus can also compress the veins slowing blood flow, which can lead to blood clots.

Blood clot risk increases for the six weeks following delivery and is highest in women who have had a C-section.

Hormone Replacement Therapy (HRT)

Some forms of HRT, particularly those containing estrogen, can increase blood clot risk. Since HRT comes in many forms—using different combinations of hormones, including progesterone (or its synthetic form, progestin)—it's important to talk to your healthcare provider about the one that is safest for you. 

Birth Control Pills

As with HRT, many pills, patches, and rings contain estrogen, which can increase your risk. Pills containing drospirenone, a form of the hormone progestin, may increase blood clot risk compared to birth control that uses a different progestin. 

Yaz, Yasmin, Beyaz, and Safyral are birth control pills that contain drospirenone.

The overall risk of developing a blood clot is low for women who use oral contraceptives—only one in 3,000 per year.

But there is no need to panic if you are currently using a birth control method that contains drospirenone. There are many different formulations, containing different combinations of hormones.

If you have any concerns or questions, discuss the risks and benefits of using any form of birth control pills, especially if you smoke or have other risks factors for blood clots. 

Read more about how blood clots are diagnosed.

Frequently Asked Questions

  • What causes blood clots to form?

    A blood clot, or thrombus, forms as a result of thickened (coagulated) blood, which is necessary for healing skin wounds, but can prove dangerous if the clot blocks blood flow to an essential organ like the brain, heart, or lungs.

  • How do you know if you have a blood clot?

    Though symptoms may vary depending on the clot location, size, and what caused it, there are several warning signs to watch out for:

    • Skin tenderness, redness, and warmth in the area of the clot
    • Swelling
    • Dizziness
    • Numbness or weakness
    • Chest pain
    • Shortness of breath
    • Nausea or vomiting
    • Breaking out in a cold sweat
  • Why are pregnant people at risk for blood clots?

    In pregnancy, the number of platelets and clotting factors increase, which means pregnant people may be at a higher risk for developing a blood clot both during and up to six weeks after delivery.

  • What is the connection between blood clots and the birth control pill?

    People who take certain forms of the birth control pill (oral contraception) may be at an increased risk of blood clots, because estrogen and some types of synthetic progesterone may increase the activity of clotting factors. However, the risk is very low.

  • Can stress cause blood clots?

    Yes, in some cases, studies have shown that intense stress may increase the risk of a blood clot or other cardiovascular event, especially in those with pre-existing atherosclerosis.

10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Atrial Fibrillation Fact Sheet|Data & Statistics|DHDSP|CDC. Centers for Disease Control and Prevention.

  2. What is Venous Thromboembolism? | CDC. Centers for Disease Control and Prevention.

  3. Hospitalization and Blood Clots | CDC. Centers for Disease Control and Prevention.

  4. About Factor V Leiden Thrombophilia.

  5. Prothrombin thrombophilia - Genetics Home Reference - NIH. U.S. National Library of Medicine.

  6. Cohoon KP, Heit JA. Inherited and secondary thrombophilia. Circulation. 2014;129(2):254-7. doi:10.1161/CIRCULATIONAHA.113.001943

  7. American College of Obstetricians and Gynecologists' Committee on Practice Bulletins—Obstetrics. ACOG practice bulletin No. 196: Thromboembolism in pregnancyObstet Gynecol. 2018;132(1):e1-e17. doi:10.1097/AOG.0000000000002706

  8. American Society of Hematology. Blood clots.

  9. American College of Obstetricians and Gynecologists. Combined hormonal birth control: Pill, patch, and ring.

  10. von Känel R. Acute mental stress and hemostasis: When physiology becomes vascular harm. Thromb Res. 2015;135(Suppl 1):S52-S55. doi:10.1016/S0049-3848(15)50444-1

Additional Reading
  • University of California, Berkeley, School of Public Health, White Paper on Heart Attack Prevention.

  • American Heart Association. Understanding Your Risk for Excessive Blood Clotting.
  • Mayo Clinic. Hormone Therapy: Is it Right for You?
  • National Blood Clot Alliance. Know Your Risk.

By Dawn Stacey, PhD, LMHC
Dawn Stacey, PhD, LMHC, is a published author, college professor, and mental health consultant with over 15 years of counseling experience.