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Different teratogens affect development in different ways. Moreover, it is not always easy to predict the harm caused by teratogens. Generally, several principles can account for the varied effects of exposure to teratogens on prenatal development (Moore & Persaud, 2016; Sadler, 2015).

 Critical Periods. The extent to which exposure to a teratogen disrupts prenatal development depends on the stage of prenatal development when exposure occurs. That is, there are critical periods during prenatal development in which the developing organism is more susceptible to damage from exposure to teratogens. Exposure to teratogens during the germinal stage can interfere with cell division and prevent implantation; however, during this stage most women are unaware that they are pregnant and the effects of teratogens often go unnoticed. It is during the embryonic period that the embryo is most sensitive to the harmful effects of teratogens (Webster et al., 2018). Structural defects occur when the embryo is exposed to a teratogen while that part of the body is developing. As shown in Figure 3.3, each organ of the body has a sensitive period in development during which it is most susceptible to damage from teratogens such as drugs, alcohol, and environmental contaminants. Once a body part is fully formed, it is less likely to be harmed by exposure to teratogens; however, some body parts, like the brain, remain vulnerable throughout pregnancy.DescriptionFigure 3.3 Sensitive Periods in Prenatal DevelopmentSource: Levine and Munsch (2010, p. 113).

 Dose. The amount of exposure (i.e., dose level) to a teratogen influences its effects. Generally, the greater the dose and the longer the period of exposure, the more damage to development. However, teratogens also differ in their strength. Some teratogens, like alcohol, display a powerful dose–response relationship so that larger doses, or heavier and more frequent drinking, result in greater damage (Muggli et al., 2017).

 Individual Differences. Individuals vary in their susceptibility to particular teratogens based on the genetic makeup of both the organism and mother, as well as the quality of the prenatal environment. How an organism responds to a teratogen may vary with its genetic vulnerability. That is, teratogens increase the risk of defects for all organisms, but responses may vary such that some organisms show severe defects, others show more mild defects, and some may display normal development. The mother’s genetic makeup and the prenatal environment (e.g., nutrition) may also increase or decrease the likelihood of teratogenic defects.

 Complicated Effects. Different teratogens can cause the same birth defect, and a variety of birth defects can result from the same teratogen. Also, some teratogenic effects may not be noticeable at birth but instead emerge later in life. Sleeper effects refer to detrimental outcomes of exposure to teratogens and early risks that appear only later in development. For example, infants born to women who consumed diethylstilbestrol (DES), a hormone that was widely prescribed between 1945 and 1970 to prevent miscarriages, were born healthy but as adults were more likely to experience problems with their reproductive systems. Daughters born to mothers who took DES were more likely to develop a rare form of cervical cancer, have miscarriages, and give birth to infants who were premature or low birthweight (Conlon, 2017).