Any substance, infection, chemical or drug that damages or adversely affects the development of an unborn foetus is called a teratogen. Thalidomide, irradiation, syphilis and isotretinoin are all teratogens. Some teratogens (eg. german measles – rubella) are only dangerous at specific stages of the foetal development. The most dangerous stage for the action of a teratogen is from three to twelve weeks of pregnancy.
Other commonly recognised teratogens include ACE inhibitors, alcohol in excess, androgens, cytomegalovirus, diethylstilboestrol, hydrantoin, mercury, methotrexate, parvovirus, penicillamine, phenylketonuria, polychlorobiphenyls, systemic lupus erythematosus, tetracyclines, toxoplasmosis, valproic acid and warfarin. There are many other teratogens that have not been listed.