A blood clot (also known as a thrombus) is when a part of the blood thickens and sticks together. This can happen anywhere in your body, but it’s most common in veins near your heart or brain. The risk of serious complications from an otherwise minor blood clot depends on where the clot forms and how big it is.
For example, a clot in your leg is far less likely to cause major problems than one that develops near your brain. And the risk of serious complications from an otherwise minor blood clot also depends on how long it’s been there: A new or recent injury may lead to a more severe complication like deep vein thrombosis (DVT).
The bigger and older the blood clot, the more likely it will get worse—and eventually break off and move somewhere else in your body where it can do damage. A person with DVT who doesn’t receive treatment runs about a 20% chance of having another episode within two years. Individuals without symptoms remain at high risk for repeat events if they don’t take preventive measures.