Teratogens are physical or chemical agents which interfere with normal embryonic development leading to congenital malformations. Such malformations are not hereditary (in contrast to those resulting from changes in genetic material). The period of maximum teratogenic sensitivity is the embryonic. The foetus becomes less susceptible the more it grows; however teratogens can still disturb its development.
In addition to general malformations, some teratogenic agents affect particular organ systems: thalidomide affects the skeleton and limbs; X-rays cause malformation of nerve and eye tissues; rubella causes heart deformities, cataracts and deafness.
It has been shown that a very broad range of chemical agents can produce, under the proper conditions, some serious type of developmental deviation. Substances already found to be embryopathic in animals range from highly toxic substances, such as anti-tumour agents, to commonplace consumer items such as aspirin, and completely inert materials. There are 33 known and proven teratogens. Known agents include: chemical factors (methylmercury, aminopterin, thalidomide, iodine deficiency, steroid hormones with androgenic activity, carbon monoxide); infective micro-organisms (rubella virus, herpes virus, cytomegalovirus, toxoplasma, syphilis); physical factors (ionizing radiation, trauma and perhaps high temperatures).