A dental hygienist is concerned about the effects of radiation on the in utero development of her baby. During which of the following periods is the embryo most susceptible to environmental influences that could induce the formation of nonlethal congenital malformations?

a. Fertilization to 1 week of fetal life
b. The second week of fetal life
c. The third through eighth weeks of fetal life
d. The third month of fetal life
e. The third trimester of fetal life

The answer is c. Exposure of the embryo to harmful environmental factors (teratogens), such as chemicals, viruses, and/or radiation, can occur at any time. During the third through eighth weeks of embryonic life, organ systems are developing and are most susceptible to teratogens. During that time, each organ system has its own specific period of peak susceptibility. Exposure of the embryo to teratogens during the first 2 weeks of fetal life (answers a and b) generally induces spontaneous abortion and is, therefore, lethal. After the eighth week of intrauterine development (answers d and e), teratogenic exposure generally results in retardation of organ growth rather than in new structural or functional changes.