Congenital disorder, also known as congenital disease, birth defect or anomaly, is a condition existing at or before birth regardless of cause. Of these diseases, those characterized by structural deformities are termed "congenital anomalies" and involve defects in a developing fetus. Birth defects vary widely in cause and symptoms. Any substance that causes birth defects is known as a teratogen. Some disorders can be detected before birth through prenatal diagnosis (screening).