AARP Eye Center
Blood, by definition, is a liquid, but if you nick your finger while chopping vegetables, you want that liquid to solidify pretty quickly. That's where coagulation — aka blood clotting — comes in: When everything is working properly, platelets rush to a wound and create a plug; then a threadlike protein called fibrin forms a mesh to stabilize it. As you heal, the clot slowly dissolves, and normal blood flow eventually resumes in the injured area.
Sometimes, however, blood clots form where they're not supposed to. Developing an inappropriate clot can be a mere nuisance, or it can put your life in danger. The threat level largely depends on where the clot occurs, says Maissaa Janbain, M.D., associate director of the Louisiana Center for Bleeding and Clotting Disorders at the Tulane University School of Medicine.
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Clots that form in the superficial veins just under the surface of the skin are rarely dangerous. If you have varicose veins, for instance, you probably have some superficial blood clots, and although they might be painful or tender, they're unlikely to seriously harm you. Other types of blood clots, however, have the potential to be extremely dangerous. You're far more likely to develop one of these troublesome clots if you carry a genetic mutation that makes your blood clot too easily. (If you have a family history of clotting problems, your doctor might order blood tests to check.)
Pregnant women are also at greater risk, as are women on hormonal medication (such as birth control pills, hormone therapy for menopause symptoms, or hormonal breast cancer treatment). Being inactive for extended periods because of, say, surgery or long-distance travel, also ups the risks of dangerous clots. Obesity, smoking and certain autoimmune disorders are notable risk factors as well.
People who develop dangerous clots, says Janbain, “often have an underlying problem, like a genetic mutation, as well as an acquired one, like being on hormonal birth control."