It can be very dangerous. 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.
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 the anti-clotting protein C in the blood isn't doing its job properly. In some people, the blood clots too much because their body doesn't make enough anti-clotting protein C or protein S, or they have antithrombin III that doesn't work. Some people have an extra protein in their blood that causes too much clotting.