| Category | Description |
| A | Adequate, well-controlled studies in pregnant women have not shown an increased risk of fetal abnormalities. |
| B | Animal studies have revealed no evidence of harm to the fetus, however there are no adequate and well-controlled studies in pregnant women.or Animal studies have shown an adverse effect, but adequate and well-controlled studies in pregnant women have failed to demonstrate a risk to the fetus. |
| C | Animal studies have shown an adverse effect and there are no adequate and well-controlled studies in pregnant women. or No animal studies have been conducted and there are no adequate and well-controlled studies in pregnant women. |
| D | Studies, adequate well-controlled or observational, in pregnant women have demonstrated a risk to the fetus. However, the benefits of therapy may outweigh the potential risk. |
| X | Studies, adequate well-controlled or observational, in animals or pregnant women have demonstrated positive evidence of fetal abnormalities. The use of the product is contraindicated in women who are or may become pregnant. |
Drugs that fall under category A have had several well-controlled studies that found no harmful effects or increase in birth defects. These drugs have all had studies conducted in pregnant woman with positive results. Very few drugs fall into this category. Prenatal vitamins receive a category A rating.
Drugs assigned a category B rating are not likely to pose a threat to the fetus from the evidence in animal studies, but no well-controlled studies have been performed in pregnant women. However, a drug may also receive a category B rating if animal studies have shown evidence of fetus damage but the same drug tested on pregnant women posed no threat.
A category C rating is given to drugs that have been shown to be harmful in animal studies but no studies have been conducted on pregnant humans. Drugs may also receive a category C rating if the drug was not studied in animals and there isn't enough evidence from studies in pregnant humans. This implies that the drug may or may not be safe to take.
Drugs receive a category D rating when the drugs have been tested in well-controlled or observational (not controlled) studies, which resulted in harm to the unborn baby. In some cases these drugs may still be given if the benefits to the mother outweigh the risks to the baby (for example, cancer treatment).
A Category X rating is assigned to drugs that should never be used during pregnancy, as there are no benefits that would exceed the potential risk.