The FDA (U.S. Food and Drug Administration – one of the most stringent drug review agencies in the world) classifies pregnancy drugs into five classes: Class A, B, C, D, and X, based on the different levels of teratogenic risk they pose to animals and humans, for clinical reference in selecting safe drugs for use during pregnancy. Class A: In controlled studies of women, no risk to the fetus in the first, second and third trimesters of pregnancy was found, and the possibility of fetal harm was low. Class B: There is insufficient or no evidence of fetal risk in controlled studies of pregnant women in the first, second, and third trimesters of pregnancy. Class C: Drugs have been shown to cause fetal malformation or death in animal studies, but there are no controlled studies in women and the potential risk to the fetus must be carefully weighed when using the drug. Class D: Evidence of fetal risk in humans is conclusive and must be weighed in favor of use in pregnant women, only when the woman’s life is at risk or when a serious illness makes its use indispensable. Class X: Studies in animals or humans have shown that the drug can cause fetal abnormalities. Contraindicated in women who are pregnant or may become pregnant. It is important to note that the above criteria are for reference only and that safe use of medication depends not only on the type of medication but also on the dose of the medication. Therefore, the use of any medication during pregnancy should be decided by a medical professional and should be strictly adhered to by pregnant women.