Blood clots are made when substances in the blood thicken and form a semisolid mass. This process may be triggered by an injury. Sometimes it occurs inside blood vessels that don't have an obvious injury.

Once these clots form, they can travel to other parts of the body where they can cause harm. There are a variety of factors and conditions that can cause troublesome blood clots, and there are some serious conditions that are associated with blood clots. These include:

1. Antiphospholipid syndrome
2. Arteriosclerosis / atherosclerosis
3. Cancer
4. Certain medications, such as oral contraceptives and hormone therapy drugs
5. Coronavirus disease 2019 (COVID-19)
6. Deep vein thrombosis (DVT)
7. Factor V Leiden
8. Family history of blood clots
9. Heart arrhythmia
10. Heart attack
11. Heart failure — a condition in which the heart doesn't pump blood as well as it should.
12. Obesity
13. Peripheral artery disease (PAD)
14. Polycythemia vera — a type of blood cancer.
15. Pregnancy
16. Prolonged sitting or bed rest
17. Pulmonary embolism
18. Smoking.
19. Stroke
20. Surgery