Teratology in the 20th century: environmental causes of congenital malformations in humans and how they were established.

In medicine, it was once customary to divide disease into two opposing classes: congenital and acquired. For teratology, however, these two terms need not be contrasting at all. The difficulties of discovering the causes of abnormal fetal development did not impede—perhaps stimulated—efforts to devise systems of etiological classification. Classification is done on the basis of cause, type, and pathogenesis. In all the annals of human teratology, the most notorious example of variable malformation frequency belongs to certain defects of the central nervous system (CNS). These comprise a set of malformations characterized by the failure in early embryogenesis of the neural tube to close. Additional to “inherent” or biological causes of variations are those of conventional ascertainment bases. A largely unavoidable source of faulty estimation of malformation frequency is incomplete or disinterested recording of information on hospital charts and birth and death and other registry certificates. The search for the causes of human congenital malformations has been the principal objective of teratologic investigation since the 20th century. Only through studies having this objective will malformations, to the extent possible, be prevented. The interests and professional backgrounds of investigators engaged in this quest have been varied—clinical, genetic, epidemiologic, experimental, ecologic, applied—but in this striving, they have often interacted and been mutually fruitful.