The FDA pregnancy risk categories are used to classify the potential risks that drugs may pose to the developing fetus when taken during pregnancy.

- Category X is assigned to drugs that have demonstrated significant risk of fetal abnormalities or adverse developmental outcomes based on human or animal studies.
- These drugs have risks that clearly outweigh any possible benefit, and therefore, they are contraindicated in pregnancy.
- This means they should never be used by pregnant women due to their known potential for causing birth defects or fetal harm.

To clarify the other categories:
- Category B: Animal studies have not demonstrated a risk to the fetus, but there are no adequate human studies, or animal studies showed adverse effects not confirmed in humans.
- Category C: Animal studies have shown adverse effects on the fetus, but there are no adequate well-controlled human studies; drugs should be used only if the potential benefit justifies the risk.
- Category D: There is evidence of human fetal risk based on adverse reaction data, but the benefits from use in pregnant women may be acceptable despite the risk in serious or life-threatening situations.

Therefore, Category X represents the highest level of risk, where the use of the drug is contraindicated in pregnancy due to clear evidence of fetal harm.

Reference: Goodman & Gilman's The Pharmacological Basis of Therapeutics, 13th Edition, Volume 2, Chapter 37: Drugs in Pregnancy and Lactation, Page 945