A perspective on the significance of maternally mediated developmental toxicity.

The significance of maternally mediated developmental toxicity has been controversial from both a biological and a regulatory point of view. The open literature has at times been interpreted to mean that a number of the effects seen in fetuses from dams exposed to maternally toxic doses of chemicals were secondary consequences of maternal toxicity rather than direct effects on the conceptus. Recent experimental studies, however, indicate that although certain relatively species-specific manifestations of developmental toxicity may at times be maternally mediated, most are not. On occasion, even severe maternal toxicity can apparently occur without causing readily discernible effects on the embryo/fetus. The most important concern of a regulatory agency with regard to developmental toxicity is the possibility of the causation of significant, irreversible harm to the offspring. In practical terms, the margin of safety for exposure to a developmental toxicant is of much more importance than whether or not the agent's effects are maternally mediated. For protection of the unborn, it is obviously the end result that matters, regardless of the mechanism. Safeguarding the conceptus from specific developmental toxicants (i.e., agents with relatively high A/D ratios) requires the use of safety factors based on the developmental toxicity NOEL. Protecting the conceptus against agents with A/D ratios near unity could be based on the maternal toxicity NOEL, however, as the true NOEL for developmental toxicity may be near that for the mother, but the adult NOEL is likely to be more readily determinable.