teratology, branch of the biological sciences dealing with the causes, development, description, and classification of congenital malformations in plants and animals and with the experimental production, in some instances, of these malformations. Congenital malformations arise from interruption in the early development of the organism. Malformations in human infants, for example, may occur because the infant’s genotype contains mutant genes or includes an abnormal number of chromosomes; they also may occur if early in pregnancy the mother has had German measles (rubella), has taken some injurious drug, or has been exposed to an injurious dosage of radiation. Experimental studies suggest similar types of factors can cause malformations in animals and plants.