Drugs can act directly on the embryo and affect it. Indirectly, through biotransformation into metabolites, they can also have teratogenic effects. Pregnancy is a special physiological period, during which all systems have obvious adaptive changes, 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 have effects on it; they can also have teratogenic effects indirectly through biotransformation into metabolites. Therefore, maternal medicine should be used reasonably. I. Effects of drugs on different periods of pregnancy Pre-implantation – is the period between the fertilization of the egg and the implantation of the fertilized egg in the endometrium, which refers to the period of 2 weeks after fertilization. In this period, the fertilized egg is not yet in direct contact with the maternal tissue and is still in the oviductal cavity or uterine secretion, so the drugs used in the preimplantation period have little effect on it. The necessary condition for the drugs to affect the blastocyst is that the drugs must enter the secretion in a certain amount to work, and if the drugs are extremely toxic to the blastocyst, it can cause very early abortion. The late stage of blastocyst is the teratogenic period of drugs, when the embryo and fetal organs are highly differentiated, rapidly developing and constantly forming, firstly, the heart and brain begin to differentiate and develop, followed by the eyes and limbs. At this time, the toxicity of drugs used by pregnant women can interfere with the normal differentiation of embryonic and fetal tissue cells, and any part of the cells receiving 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 be. From 12 weeks of gestation to delivery – the organs of the fetus are formed and the teratogenic effect of drugs is obviously reduced. However, some drugs may affect organs that are not yet fully differentiated, such as the reproductive system, while the neurological system may remain affected by drugs because it continues to differentiate and develop throughout pregnancy. During childbirth – the use of drugs should take into account the impact on the newborn baby to be born. Maternal medication principles 1. must have clear indications to avoid unnecessary use of drugs. 2, must be under the guidance of doctors, do not use drugs without authorization. 3.Able to use a drug, avoid combined use of drugs. 4, can use the efficacy of the more certain drugs, to avoid the use of new drugs is difficult to determine whether there are adverse effects on the fetus. 5, can use small doses of drugs, avoid using large doses of drugs. 6, strict control of drug dose and duration of medication, pay attention to the timely discontinuation of drugs. 7.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 the drugs should be used later. The U.S. FDA has classified the harmfulness of drugs to the fetus into five levels: A, B, C, D and X according to the teratogenic situation of drugs to the fetus. Class A: After clinical control studies, it cannot be confirmed that the drugs have harmful effects on the fetus in early and middle and late pregnancy, 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 effects on fetus were observed by animal experimental studies. 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 experiments have shown that there are 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 risks to the fetus. For example, gentamicin, isoproterenol, isoniazid, etc. Class D: There is sufficient evidence of fetal risk. 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 experiments have proven to cause fetal malformations. Contraindicated during pregnancy or in women who may become pregnant. E.g. methotrexate, vinblastine, etc. Class C, D and X drugs are contraindicated in the first 12 weeks of pregnancy.