What does the FDA pregnancy category system mean for prescribing?

The FDA replaced the traditional letter categories A through X with narrative labeling in 2015 under the Pregnancy and Lactation Labeling Rule, or PLLR. However, the old system is still widely referenced on exams. Category A had controlled studies showing no fetal risk. Category B had no evidence of risk in humans. Category C had risk not ruled out. Category D had positive evidence of fetal risk but benefits may outweigh risks. Category X was contraindicated because fetal risk clearly outweighs any benefit.