nancies (Fretts et al. 1995). Chromosomal aberrations occur in approximately 1% of live births (Oakley 1986). Severe mental retardation is an example of a functional deficit that might not be recognized at birth but is recognized in approximately 0.4% of children before 15 years of age (Velie and Shaw 1996).

Developmental defects are often defined as those originating in the embryo and fetus, that is, in the prenatal period. A developmental toxicant is then a toxic agent or condition to which the pregnant mother is exposed. However, development goes on throughout the life cycle and includes for example, the continued growth and differentiation of the nervous, skeletal, and reproductive systems in the juvenile and adolescent, and the continuous renewal of cells of the skin, gut lining, and hematopoetic system of the adult. Thus, it is arbitrary to define developmental toxicants only as those affecting the embryo or fetus through maternal exposure in the period of pregnancy. In this report, the committee emphasizes developmental toxicants to which the mother may be exposed in the prenatal period. However, the division is not sharp, and it is to be expected that toxicant