A framework for categorizing medications based on their known hazards when used during pregnancy. Category A: Rigorous investigations involving human subjects have shown no discernible risk to the developing fetus. Category B: Research conducted on animals suggests no fetal risk, although there are no equivalent studies on humans; alternatively, adverse effects observed in animals have not been observed in well-controlled studies on humans. Category C: Insufficient data from either human or animal studies; either adverse effects on fetal development have been observed in animal studies but no human data is available, or there is a lack of adequate research altogether. Category D: Existing evidence points to potential risks to the fetus, but the benefits of using the medication outweigh these risks. Category X: Substantial evidence of fetal risk, where the potential dangers to the developing fetus far outweigh any potential benefits.