Excerpt: The eggs which will form her offspring are already present in the ovaries of the female human fetus. Thus, these cell can exist without further replication for over 40 years prior to being fertilized, and the child formed from that union of gametes can live for a century or more afterward. The incredible changes occurring during the entire life cycle obviously affect susceptibility to environmental exposures. Although this is intuitive and well recognized, the affect of life stage on susceptibility has been incorporated into human health risk assessments in only a limited way. The rapid and profound changes occurring during intrauterine life alter the susceptibility of the developing conceptus on a daily or even hourly basis, and knowledge of this changing susceptibility is inherent in current study designs for developmental toxicity.