Teratogenesis is defined as the dysgenesis of fetal organs as evidenced either structurally or functionally (mental retardation). A teratogen is an agent that can produce a permanent alteration of structure or function in an organism exposed during embryonic or fetal life. Many agents can produce a teratogenic effect under some circumstances. Types of teratogens include radiation, infections, drugs, metabolites and environmental chemicals. The developmental stage of the embryo or fetus at the time of exposure is critical. During conceptions and for about 2 weeks thereafter, one damaged cell can be replaced by another, and normal development will usually ensue, although the embryo will not survive if too many cells are damaged. This is known as the “all-or-nothing” period. The subsequent period of organogenesis, from 18 to 60 days after conception (about 4.5-11 weeks after LMP) is the time of greatest sensitivity or critical period for teratogenic exposure. Fetal exposure later in gestation usually does not produce gross- structural abnormalities, although there are some exceptions. Dose and route of exposure are critical factors in teratogenesis. Teratogenic effects occur only when the dose exceeds a certain threshold. A chronic exposure is usually of more concern than a single exposure, given similar doses. There is unlikely to be a teratogenic effect when the exposure occurs by a route that does not permit systemic absorption. The teratogenicity of an exposure is also determined by both the maternal and fetal genotypes. This may result from differences in cell sensitivity, placental transport, metabolism, receptor binding and distribution.