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Birth defects are common. Of all newborns, 2 to 3 percent have a major congenital abnormality detectable at birth (Cragan, 2009; Dolk, 2010). Some medications undoubtedly pose significant risk to the developing embryo or fetus (Table 8-1). However, 80 percent of birth defects do not have an obvious etiology, and of those with an identified cause, nearly 95 percent of cases have chromosomal or genetic origins (Feldkamp, 2017). The U.S. Food and Drug Administration (FDA) (2018) estimates that <1 percent of all birth defects are caused by medications (Fig. 8-1).

TABLE 8-1Selected Teratogenic and Fetotoxic Agents

Etiology of birth defects. Known and unknown causes of 5504 birth defects from a population-based review of 270,878 births.

That said, significant concern surrounds medication use in pregnancy. This is because many pregnant women are prescribed medications and because safety data are often lacking. Investigators from the National Birth Defects Prevention Study found that women take an average of two to three medications per pregnancy and that 70 percent use medication in the first trimester (Mitchell, 2011). A population-based review from Canada found that 65 percent of women fill ≥1 prescription during pregnancy (Leong, 2019). Between 2010 and 2019, the FDA approved 290 new drugs, and only 11 percent had human data related to pregnancy (Byrne, 2020).


The study of birth defects and their etiology is termed teratology, derived from the Greek teratos, meaning monster. A teratogen may be broadly defined as any agent that acts during embryonic or fetal development to produce a permanent alteration of form or function. Thus, a teratogen may be a medication or other chemical substance, a physical or environmental factor such as heat or radiation, a maternal metabolite as in diabetes or phenylketonuria, or an infection such as cytomegalovirus.

Strictly defined, a teratogen causes structural abnormalities. A hadegen—after the god Hades—is an agent that interferes with normal maturation and function of an organ. A trophogen is an agent that alters growth. Substances in the latter two groups typically affect development in the fetal period or postnatally and are thus fetotoxins rather than teratogens.

Criteria for Determining Teratogenicity

The guidelines shown in Table 8-2 were proposed by Shepard (1994) as a ...

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