Teratogen refers to any agent that causes a structural abnormality following fetal exposure during pregnancy. Seldom, if ever, have teratogens been identified following designed epidemiologic studies. Usually an increased prevalence of a particular birth defect leads to the discovery of a teratogenic agent. For instance, the sudden appearance of several cases of a rare disorder can also raise suspicions of teratogenicity. Cases of phocomelia in the early 1960's in Germany and Australia led to the identification of thalidomide as a human teratogen. Thalidomide was used to treat morning sickness, resulting in exposure at the stage in development when the embryo is most vulnerable, the first trimester. Teratogenic agents include: infectious agents (rubella, cytomegalovirus, varicella, herpes simplex, toxoplasma, syphilis, etc.); physical agents (ionizing agents, hyperthermia); maternal health factors (diabetes); environmental chemicals (organic mercury compounds, polychlorinated biphenyl or PCB, herbicides and industrial solvents); and drugs (prescription, over- the-counter, recreational). It may appear as though there are more suspected teratogens than were apparent a generation ago. This may be because there has been an increase in the number of synthetic chemical compounds in use or possibly the clinical recognition of subtle malformations as teratogenic effects. Examples of the latter would be fetal alcohol syndrome, fetal hydantoin syndrome, fetal trimethadione syndrome, fetal warfarin syndrome and smoking associated with low birth weight infants.