Growing possible effects, upon ' the developing fetus, of drugs prescribed during pregnancy was accentuated by the thalidomide tragedy. If a drug is to be prescribed for the pregnant woman, three basic points should be considered: first, the known toxic effects; second, the possibility that the drug might pass the placental barrier to a significant degree; and third, the stage of development of the fetus.
The toxic effects of most drugs are well known. Not as well defined are the ability of a particular drug to cross the placental barrier and the drug's fetal toxic level. What may be a relatively safe blood level or tissue content in the mother may possibly be toxic for the fetus. Also, a level which is relatively safe for the fetus in the last trimester may be potentially dangerous during the first trimester when the fetus is undergoing a more rapid and fundamental development.
THE PLACENTAL BARRIER AND DRUGS. JAMA. 1964;190(4):392-393. doi:10.1001/jama.1964.03070170133026