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Safe Drugs for the Fetus
July 2002

Q. Can you tell me what type of drugs (not specific drugs, but maybe drug classes) are least likely to cross the placental barrier and affect the developing fetus? Do all drugs affect the fetus?

A. The question of medication used during pregnancy is an important one. If you look up a medication in a book such as the "Physician's Desk Reference" which is quite comprehensive, you will find that there is a section on "useage during pregnancy" but the information about this is quite limited. The book will often state things like "safety in pregnancy has not been established" due to the limitations of such testing.

The safety data is based on animal models and translating that into predictions for human safety is very speculative. Some drugs are clearly unsafe in pregnancy but the data for most is very incomplete. The FDA has a system for safety as follows:

CATEGORY A: No fetal risk found from controlled studies in humans. Few drugs fit this description. An example would be prenatal vitamins.
CATEGORY B: no fetal risks found in animal studies, but no sufficient studies exist in humans.
CATEGORY C: Either no adequate studies in animals or humans, or there have been shown adverse fetal effects in animal studies but no direct studies in humans.
CATEGORY D: There is evidence of fetal risk, but benefits may outweigh the risks in certain situations.
CATEGORY X: Proven fetal risks exceed any benefit. Example: Accutane.

Most obstetricians are quite conservative on the issue, meaning they try to minimize the use of medication for the reasons discussed above.



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