A congenital anomaly is a structural abnormality of any type that is present at birth. Congenital anomalies may be caused by genetic factors or by environmental factors (infection agents, environmental chemicals and drugs) that cause disruptions during prenatal development, but most common congenital abnormalities result from a complex interaction between genetic and environmental factors. During the first two weeks of development, teratogenic agents, usually, kill the embryo rather than cause congenital anomalies, During the organogenesis period, teratogenic agents disrupt development and may cause major congenital anomalies. During the fetal period teratogens may produce morphological and functional abnormalities, particularly of the brain and eyes. Mental retardation may result from high levels radiation and infectious agents.