Birth defects may occur due to genetic/ chromosomal abnormalities or environmental factors or a combination of both. Congenital anomalies may be classified as malformations, disruptions, deformities and grouped as a sequence or a syndrome. Chromosomal abnormalities may be structural or numerical. Some examples of environmental factors include infections like rubella and CMV, radiation, prenatal chemical/drug exposure of thalidomide and antiepileptic drugs, hormones such as diethylstilbesterol, maternal disease such as diabetes and PKU and hypoxia.