A heart murmur may be an abnormal, extra sound during the heartbeat cycle made by blood moving through the heart and its valves. Heart murmurs may be caused by defective heart valves. Murmurs also can be caused by conditions such as pregnancy, fever, anemia or thyrotoxicosis (a diseased condition resulting from an overactive thyroid gland). Innocent heart murmurs are sounds made by the blood circulating through the heart's chambers and valves or through blood vessels near the heart. Sometimes called functional or physiologic murmurs, Innocent murmurs are common in children and are harmless.