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During the first half of the 20th century, the field of teratology (the study of birth defects) was represented sparsely in the medical literature. Pediatricians who graduated from medical school in the 1950s were taught that the correct number of human chromosomes was 48, until the number was reported accurately as 46 by Tijo and Levan in 1956. (1) Gregg had described the teratogenicity of rubella virus infection during pregnancy in 1941. (2) The teratogenic risk of the folic acid antagonists (aminopterin) was established in humans, (3)(4) and experimental studies indicated that nutritional deficiencies could produce congenital malformations (CMs) in animals. (5)(6)(7)

In the 1950s, teratologists were aware that only a small percentage of birth defects were caused by drugs, chemicals, and physical agents. (5)...

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