The formation of a thrombus (blood clot) in an undamaged blood vessel. A thrombus that forms within an artery supplying the heart muscle (coronary thrombosis) is the usual cause of myocardial infarction. A thrombus in an artery of the brain (cerebral thrombosis) is a common cause of stroke. Thrombi sometimes form in veins, either just below the skin or in deeper veins (see thrombosis, deep vein). In arteries, thrombus formation may be encouraged by atherosclerosis, smoking, hypertension, and damage to blood vessel walls from arteritis and phlebitis. An increased clotting tendency may occur in pregnancy, when using oral contraceptives, or through prolonged immobility. An arterial thrombosis may cause no symptoms until blood flow is impaired. Then, there is reduced tissue or organ function and sometimes severe pain. Venous thrombosis may also cause pain and swelling. Diagnosis is made by doppler ultrasound. In some cases, angiography or venography may also be used. Treatment may include anticoagulant drugs or thrombolytic drugs, nonsteroidal anti-inflammatory drugs, and antibiotic drugs. In life-threatening cases, thrombectomy may be needed.