However, your blood shouldn't clot when it's just moving through your body. Sometimes the thrombus can travel in the bloodstream and get stuck in your lungs. A pulmonary embolus can be life-threatening. A clot that blocks a blood vessel in the brain can cause a stroke. A clot in a blood vessel in the heart can cause a heart attack.
Blood clots can cause some women to have miscarriages.
In addition to proteins that help with clotting, your blood also has some proteins that prevent clotting. Usually, these anti-clotting proteins are strong enough to stop clotting in the veins when it shouldn't happen. However, certain situations or risk factors can make it easier for your blood to clot too much. Some people are born with a tendency to develop clots. Most of the time, increased clotting happens because protein C in the blood isn't doing its job properly.