congenital heart disease, any defect in the heart present at birth. There is evidence that some congenital heart defects are inherited, but the cause of most cases is unknown. One known cause is infection of the mother with the rubella (German measles) virus during the first trimester of pregnancy. Among the most common congenital heart disorders are malformations in the valves and the persistence of structures that are normally closed off at birth, i.e., the ductus arteriosis (the fetal blood vessel that shunts blood from the pulmonary vein to the aorta, bypassing the heart) and the foramen ovale (the opening between the left and right atria of the fetal heart). If the malformation is severe, it will produce various symptoms of insufficient heart function, such as cyanosis (a bluish tinge to the skin), dyspnea (difficulty in breathing), fatigue, and abnormal heartbeat; valvular deformities predispose the patient to bacterial infection of the endocardium (see endocarditis). Less severe malformations may not produce noticeable symptoms until later in life, and some may not require any medical attention. Many congenital heart defects that are debilitating can be corrected surgically. Other congenital anomalies, such as Down syndrome, are present in about 20% of cases of congenital heart disease. See also heart disease.