Teratogens are substances that produce neurological and physical malformations in developing human fetuses. The word comes from the Greek teras, meaning "malformation" or "monstrosity." Certainly, ever since the first malformed baby was born, people have wondered what causes birth defects. Most early explanations referred to mystical forces: the influence of celestial bodies, divine intervention, even conception during the woman's menstrual period. The theory of maternal impression was widely accepted from the seventeenth to the early twentieth centuries. According to this concept, a specific, strong impression on the mother during pregnancy would produce a corresponding specific birth defect in her child; for instance, a mother who was startled by a hare might give birth to a child with a harelip.

During the nineteenth and early twentieth centuries, however, a number of researchers showed that exposure of animals (such as chickens and fish) to certain physical and chemical substances would...