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Teratogens and teratogenesis: general principles of clinical teratology.

Numerous factors hinder our ability to recognize fully human teratogens. Among these are the limitations of animal and epidemiologic studies, the lack of understanding of the mechanisms of action of most teratogens, and the variability in expression of the clinical manifestation. Dose and timing of exposure, interactions with other environmental agents, and host susceptibility influence this variable expressivity. Recent studies suggest the genetic constitution of the mother and the fetus play a central role in the teratogenic response. Techniques currently being developed may help in a near future to identify susceptible individuals and to prevent specific types of drug-induced birth defects.