Here is a cheat sheet (but don’t depend on this, check the website directly for more information) and always check with your doctor.
Category A: Animal and human studies have established little or no risk (no drug is completely safe)
Category B: Animal studies have shown little or no risk.
Category C: Animal studies have shown adverse effects on the fetus but there are cases where benefits outweigh the risks
Category D: Positive evidence of adverse fetal risk. In some limited cases benefits still outweigh risks.
Category X: Demonstrated fetal abnormalities. The risks clearly outweigh the benefits.