With the opening of the second-child policy, more and more senior pregnant women choose to have a second child, hoping to have a healthy baby, and it is especially important to use medication rationally during pregnancy to achieve optimal fertility. Pregnancy is a special physiological period, during which there are obvious adaptive changes in various systems, and the pharmacokinetic and pharmacodynamic changes of drugs in pregnant women are also significantly different from those in non-pregnant period; drugs can act directly on the embryo and affect it; they can also have teratogenic effects indirectly through biotransformation into metabolites. The maternal metabolic state, fetal growth and development, and changes in placental function during pregnancy can affect the absorption, distribution, metabolism, and excretion of drugs, and have different degrees of impact on drug toxicity. All pregnant women should use drugs rationally. The preimplantation period is the period between fertilization of the egg and the time before the fertilized egg is implanted in the endometrium, which refers to the 2 weeks after fertilization. The fertilized egg is not yet in direct contact with maternal tissues and is still in the oviductal cavity or uterine cavity secretion, so the preimplantation period has little effect on the drug. The necessary condition for the drug to affect the blastocyst is that the drug must enter the secretion in a certain amount in order to work. The teratogenic period of drugs is from about 12 weeks after the late blastocyst implantation, when the embryo and fetal organs are highly differentiated, rapidly developing and constantly forming. The toxicity of drugs used by pregnant women at this time can interfere with the normal differentiation of embryonic and fetal tissue cells, and any part of the cells affected by drug toxicity may cause malformation of a certain part of the tissue or organ. The earlier the toxic effects of the drug appear, the more serious the malformation may occur. After 12 weeks of gestation and until delivery, the organs of the fetus are formed and the teratogenic effect of drugs is obviously reduced. However, some drugs may affect the organs that have not yet been fully differentiated, such as the reproductive system, while the neurological system continues to differentiate and develop throughout pregnancy, so the effects of drugs on the neurological system can always exist. Maternal medication principles 1. must have clear indications, avoid unnecessary use of drugs; 2. must be under the guidance of physicians, do not use drugs without authorization; 3. can use a drug, avoid the combination of drugs; 4. can use the more certain efficacy of drugs, avoid the use of new drugs that are difficult to determine whether there are adverse effects on the fetus; 5. can use small doses of drugs, avoid large doses of drugs; 6. strictly control the drug dose and drug duration, pay attention to the timely discontinuation of drugs; 6. If the condition of early pregnancy allows, try to postpone the use of drugs until the middle and late pregnancy; 8. If the condition requires the application of teratogenic drugs harmful to the embryo and fetus in early pregnancy, the pregnancy should be terminated first, and then the drugs should be used. The US FDA has classified the harmfulness of drugs to the fetus into 5 levels: A, B, C, D and X according to the teratogenic condition of the fetus. Class A: The drugs cannot be confirmed to have harmful effects on the fetus in the early and middle and late stages of pregnancy through clinical control studies, and they have the least possibility of harming the fetus and are non-teratogenic drugs. Such as moderate amount of vitamins. Grade B: No harmful effect on fetus was observed by animal test study. No clinical controlled trials, no evidence of harm was obtained. Can be used under physician’s observation. Such as penicillin, erythromycin, digoxin, insulin, etc. Grade C: Animal tests have shown adverse effects on the fetus. Since there is no clinical controlled trial, the drug should only be used with caution after fully weighing the benefits to the pregnant woman, the potential benefits to the fetus and the harms to the fetus. For example, gentamicin, isoproterenol, isoniazid, etc. Class D: There is sufficient evidence of fetal harm. Consider use only if the pregnant woman has a life-threatening or serious illness and other drugs are ineffective. For example, streptomycin sulfate, etc. Grade X: Animal and human experimental studies confirm that it causes fetal malformations. Contraindicated during pregnancy or in women who may become pregnant. E.g. methotrexate, vinblastine, etc. Class C, D, and X drugs should not be used during the first 12 weeks of pregnancy.