Most deep-vein blood clots happen in the lower leg or thigh. They're more likely to happen if you're inactive for long periods, like on a long flight or car ride. You're also at risk if you're overweight, or you smoke, or take certain medications.
There's a chance a clot could break off into your bloodstream and travel to an artery in the lungs. If so, it could block blood flow. This is a serious condition called pulmonary embolism.