Drug and Chemical Exposures in Pregnancy

Pregnant women frequently ask about the effect of drug and other chemical exposures on the unborn infant, whether environmental, OTC, or prescription. Many of these are everyday exposures at the workplace, in the community, or as a result of medical treatment and management.

Figure 21-4 Transabdominal ultrasound assessment of nuchal translucency (NT). The measurement in this figure is normal. An increased measure (>3 mm) is associated with an increased risk of chromosomal abnormalities, complex congenital heart disease, and other genetic syndromes.

The consequences can range from the most innocuous to actually jeopardizing the pregnancy. The physician should be prepared to answer these pregnancy-related questions and advise their patients appropriately.

The consequences of a chemical exposure may be related to the nature of the agent and the timing, dose, and duration of the exposure. The effect can range from minor morphologic abnormalities and growth deficiency to severe malformation and loss of the pregnancy. Thalidomide has obvious effects on the fetus, causing a third of fetuses to have limb reduction defects. Furthermore, the effect of a drug may be subtle or delayed (Welch et al., 1993). Diethylstilbestrol (DES) is associated with the development of uterine structural abnormalities and clear cell carcinoma of the vagina in daughters whose mothers took this medication.

Much of the knowledge about chemical exposures and effects on reproduction and fetal development comes from research on experimental animals. This poses great uncertainty given genetic variability and species-specific responses. Given these limitations, the health care provider must carefully weigh the evidence before using a drug. To help the practitioner classify a drug for use, the U.S. Food and Drug Administration (FDA, 1980) developed a risk factors index. The abbreviated definitions of these categories are as follows: Category A: Controlled studies in women fail to demonstrate a risk to the fetus.

Category B: Animal reproduction studies have not demonstrated a fetal risk, but there are no controlled studies in pregnant women; or animal reproduction studies have shown adverse effect that was not confirmed in controlled human studies.

Category C: Studies in animals have revealed adverse effects on the fetus, and there are no controlled studies in women. Drugs should be given only if the potential benefit justifies the potential risk to the fetus.

Category D: There is positive evidence of human fetal risk, but the benefits from use in pregnant women may be acceptable.

Category X: Studies in animals or humans have demonstrated fetal abnormalities, or there is evidence of fetal risk based on human experience. The drug is contraindicated in women who are or who may become pregnant.

Table 21-6 Drugs and Exposures in Pregnancy

This classification is an oversimplification, and the individual practitioner will need to weigh the available data in the management of the pregnant woman. Few absolutes are possible in the field of human teratology; however, current recommendations for common drug categories in pregnancy are summarized in Table 21-6. It is important to emphasize that it is difficult to demonstrate an actual cause-effect relationship between a specific drug and an adverse pregnancy outcome. At no time, however, should a drug be considered safe because no data exist.

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