Title 21, Part 201.57 (9)(i) of the Code of Federal Regulations lists specific requirements regarding the labeling of drugs with respect to their effects on pregnant populations, including a definition of a "pregnancy category".
To supplement this information, the FDA publishes additional rules regarding pregnancy and lactation labeling.
However, some of these substances are not subject to drug labeling laws, and are therefore not assigned a "Pregnancy Category" per 21 CFR 201.57.
[4] The system, as outlined below, was developed by medical and scientific experts based on available evidence of risks associated with taking particular medicines while pregnant.
Categorisation does not indicate which stages of fetal development might be affected and does not convey information about the balance between risks and benefits in a particular situation.