Classifying Drugs for Use During Pregnancy
Identifying Medications That may Harm a Pregnant Woman or Her Fetus
Sep 27, 2009
A variety of medications (or their metabolites) can pass through a human placenta and enter a developing fetus’ circulation. Some of these agents are relatively harmless; others may cause fetal harm only at certain stages of pregnancy; still others are dangerous throughout pregnancy.
Furthermore, the effects of some medications may be enhanced during pregnancy, thus increasing the risk for side effects in a pregnant woman.
One of the major components of pharmaceutical research is to ascertain a given agent’s safety for human use during pregnancy. As part of this process, pregnant laboratory animals are usually exposed to the new medication to determine whether fetal malformations, stillbirths, spontaneous abortions, premature births, or other problems arise as the result of such exposure.
Additionally, tissue cultures are exposed to the new medication to evaluate its effects on living and dividing cells.
Since it is unethical for a drug company to test new drugs on pregnant women, many drugs receive FDA approval and get onto pharmacists’ shelves before their effects on human pregnancies are completely understood, in spite of extensive pre-marketing research.
Generally, it is assumed that drugs which cause problems for laboratory animals or in tissue tests may also cause problems for humans.
Occasionally, a medication’s adverse effects are not fully realized until after the drug has been widely used. A classic and tragic example of this “after-market” research – and one that led to more stringent testing of drugs prior to their licensing – was demonstrated by thalidomide, a drug that was given to thousands of pregnant women in the late 1950s and early 60s to treat morning sickness.
Thalidomide was later shown to cause severe birth defects in human infants, and its use in pregnant women was discontinued.
FDA Pregnancy Safety Categories are Assigned to Most Medications
Currently, most medications used in the United States are assigned pregnancy safety categories to help guide their use in pregnant women. While such categorization has proved useful, it is important to remember that some medications may still receive FDA approval before their potential for fetal harm is well-defined.
FDA pregnancy safety categories are defined as:
- Pregnancy Category A: Adequate, well-controlled studies in pregnant women have failed to demonstrate risk to the fetus.
- Pregnancy Category B: Either animal findings show risk while human findings do not, or, if no adequate human studies have been done, animal findings are negative.
- Pregnancy Category C: Human studies are lacking, and animal studies are either positive for risk or are lacking, as well. However, potential benefits may outweigh the potential risk.
- Pregnancy Category D: Investigational or postmarketing data show risk to the fetus. Nevertheless, potential benefits may outweigh the potential risk.
- Pregnancy Category X: Studies in animals or humans, or investigational or post-marketing reports, have demonstrated fetal risk which clearly outweighs any possible benefit to the patient.
(Use-In-Pregnancy Ratings. Physicians' Desk Reference. Thomson PDR, Montvale, NJ. 2006:3537-40)
The FDA is considering a proposal that will eliminate current use-in-pregnancy designations and institute a more descriptive system. New drugs will be labeled with information specific to those agents,, while older medications carrying letter-designated risk profiles will gradually be phased into the new system.
Research and development for new drugs typically requires investigations to determine the possible effects of those drugs on human fetuses. Unfortunately, a medication’s potential for fetal injury may not be fully appreciated until the agent is on the market.
Therefore, some medications that may cause fetal harm can find their way into common use in spite of well-conducted pre-marketing research. A pregnant woman should consult with her physician or midwife before taking any medication, including prescription drugs, over-the-counter medicines, or herbal preparations.
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