Medication use during pregnancy is very common and most prescription drugs can be used during pregnancy and are relatively safe. Known or suspected teratogenic drugs are only a minority, and for those that are considered teratogenic, counseling should emphasize the relative risks. Exposure to a definite teratogen usually increases the risk of a pregnant woman having a child with a birth defect by only 1 to 2 percent. Some diseases, if left untreated, pose a more serious threat to the pregnant woman and the fetus than the teratogenic risk caused by exposure to drugs. Due to the specificity of clinical medicine and the ethical and moral constraints of medicine, there are insurmountable difficulties in the study of the classification of drugs used during human pregnancy. The teratogenic effects of most drugs are not known, and in order to provide therapeutic guidance, the US FDA has developed a classification system for rating the safety level of drugs used during pregnancy. It should be noted that the classification may be based on case reports or limited data from animal studies and is slow to be updated. FDA classification of drugs: Class A: Controlled studies have not found a risk to the human fetus during pregnancy, and these drugs may have minimal effect on the fetus. Class B: Animal studies have not found a risk to the animal fetus, but there is no control group for human studies; or adverse effects have been shown in animal reproduction studies, but have not been confirmed in very good human controlled studies. Category C: Animal studies have shown adverse effects on the fetus, but there are no controlled studies in humans; or there is no information from human and animal studies. This class should be used only if the potential benefit to the fetus outweighs the potential risk. Class D: There is definite evidence of risk to the human fetus, but these risks are acceptable for the benefit of the pregnant woman, for example, if the drug is life-threatening or if the condition is severe and only safe drugs are ineffective. Class X: Studies in either animals or humans have demonstrated fetal abnormalities, or are based on human experience showing a risk to the fetus, or both, and the potential risk clearly outweighs the therapeutic benefit. This class of drugs is contraindicated in pregnant women or women who may already be pregnant. There are not many drugs in this class in common use, but they are contraindicated during pregnancy because of their high teratogenic rate or their high risk to the fetus.