Birth defects can result in several ways (Table 4.02): Mutation of the genetic material may occur in the sperm or in the ova pre- or post-conception, or in the embryo or fetus. Remember that a woman has all her ova from the time of her birth! It may also occur in non-germ-line tissue throughout fetal development.
Teratogenicity occurs postconception only and involves changes in development which do not cause permanent changes in the genetic material. The teratogen must be able to move across the placental membranes. Substances may also be passed to the newborn during lactation, such as low molecular weight and/or fat soluble non-biodegradable substances. DDT or PCB's are of the latter type.
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