Legal drugs are placed in a Pregnancy Category, a system used to classify the risk of birth defects if the substance is used by a pregnant woman. Different countries have different systems of categories. The alphabetical entries in this book use the U.S. Food and Drug Administration system that was in place as the twenty-first century began.
Pregnancy Category A. Studies using pregnant women do not show birth defects caused by the drug in the first trimester, and evidence has not emerged showing risk later in pregnancy.
Pregnancy Category B. Studies using pregnant women have not been conducted, but experiments with pregnant animals fail to demonstrate birth defects. Alternatively, animal experiments have produced birth defects, but studies using pregnant women have not.
Pregnancy Category C. Animal experiments have produced birth defects, but no studies have used pregnant women to examine the potential for human birth defects. Alternatively, no animal or human studies have been conducted to determine the drug's potential for causing birth defects.
Pregnancy Category D. Studies or reports of clinical experience indicate that the drug causes human birth defects, but using the drug during pregnancy may be so important to the woman's health that fetal risk is justified (for example, no acceptable alternative therapy is available to deal with the woman's illness).
Pregnancy Category X. Animal or human studies or clinical reports indicate the drug causes birth defects, and the drug's potential benefits for a pregnant woman's health do not justify risk to fetal development (for example, acceptable therapies are available that do not involve the drug).
Often a birth defect is thought of as something apparent upon birth, but scientists have found that some problems from fetal drug exposure such as cancer do not become evident until adulthood. And even though problems might not be observed among infants of women who used a certain drug during pregnancy, that fact does not necessarily mean the drug has no impact on fetal development. Drugs discussed in this book typically affect a user's brain, suggesting that they may be particularly prone to harming fetal brain and nervous system development. Due to the course of fetal development, those effects may occur in a later stage of pregnancy than types of harm involving bones or other organs. In addition to matters affecting fetal development, pregnancy can alter a drug's effect on the woman, for example, causing a dose to last longer than in a nonpregnant woman. That, in turn, could cause a pregnant woman to receive a cumulative overdose if she uses an amount suitable for a nonpregnant woman.
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