teratogen /ter″ətəjen′/ [Gk, teras + genein, to produce] , any substance, agent, or process that interferes with normal prenatal development, causing the formation of one or more developmental abnormalities in the fetus. Teratogens act directly on the developing organism or indirectly, affecting such supplemental structures as the placenta or some maternal system. The type and extent of the defect are determined by the specific kind of teratogen, its mode of action, the embryonic process affected, genetic predisposition, and the stage of development at the time the exposure occurred. The period of highest vulnerability in the developing embryo is from about the third through the twelfth week of gestation, when differentiation of the major organs and systems occurs. Susceptibility to teratogenic influence decreases rapidly in the later periods of development, which are characterized by growth and elaboration. Among the known teratogens are chemical agents, including such drugs as thalidomide, alkylating agents, and alcohol; infectious agents, especially the rubella virus and cytomegalovirus; ionizing radiation, particularly x-rays; and environmental factors, such as the age and general health of the mother or any intrauterine trauma that may affect the fetus, especially during the later stages of pregnancy. Also called teratogenic agent. Compare mutagen. −teratogenic, adj.
