Factor V Leiden is the most common inherited blood clotting disorder. It increases the risk for deep vein thrombosis (blood clots that appear deep in the body, usually in the thigh or lower leg) and pulmonary embolism (when a blood clot travels through the blood system to the lungs). Factor V Leiden also raises the risk for problems with pregnancy, such as miscarriage. It occurs most often in children whose families are from Europe. Many people have this disorder, but most never develop harmful blood clots.