Most drugs and chemicals pose a threat to the developing fetus. An estimated 4 to 5% of developmental defects in humans result from prenatal exposure to drugs or environmental chemicals. This is particularly important, since women with irregular menstrual cycles may be exposed to teratogens and enter the sensitive period of organogenesis before pregnancy is suspected.
Gestation is generally considered to consist of three periods of development, each with differing sensitivities to chemicals. During the preimplantation or prediffer-entiation phase, expression of toxicity is an all-or-none phenomenon; damage to the embryo results in either death or no effect. Organogenesis occurs during the embryonic period (the first 3 months of pregnancy), and therefore, susceptibility to teratogenesis is high; the embryo is particularly vulnerable to teratogens on days 25 through 40. The fetal period consists of the last 6 months of gestation and is a time of reduced susceptibility to teratogenic alterations. Certain organs, such as the genitals and the nervous system, however, are still undergoing differentiation during this period. Functional impairment in tissues without marked structural damage and growth retardation is the most common effect of chemical exposure during the fetal period.
Chemicals such as 1,2-dibromo-3-chloropropane can disrupt spermatogenesis, leading to impaired reproductive function, including sterility. Men and women undergoing cancer chemotherapy with alkylating drugs are at increased risk for sterility.
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