Maternal medication principles: 1, maternal medication can use a drug to avoid combined medication; 2, can use the efficacy of the old drugs to avoid the new drugs that are difficult to determine whether there are adverse effects on the fetus; 3, can be used in small doses to avoid large doses of medication. 4. If the condition requires that the pregnant woman must apply drugs that are harmful to the embryo and fetus or even teratogenic in the early stages of pregnancy, the pregnancy should be terminated first and then the drugs should be applied. The relationship between the degree of influence of drugs and fetal gestational age: 1. The pre-fertilization period is the period from fertilization of the egg to the time when the fertilized egg is in bed with the endometrium. It is characterized by the fact that the fertilized egg is not yet in direct contact with maternal tissues and is still in the official cavity of the fallopian tube or in the secretory fluid of the uterine cavity, so the effect of drugs on pregnant women in the pre-fertilization period is minimal. 2. The effect of drugs on blastocyst: the necessary condition is that the drugs must enter the secretory fluid in a certain amount in order to work, and if the drugs are extremely toxic to the blastocyst at this stage, it can cause and early abortion. After the late blastocyst is laid until about 12 weeks, the embryo and fetal organs are highly differentiated, rapidly developing and constantly forming. 3.After 4 months of gestation, the development of fetal organs is formed, and the teratogenicity of drugs is obviously weakened, which can no longer cause a wide range of malformations, but the organs that have not yet been differentiated, such as the reproductive system, may be affected to different degrees. 4. In addition, the use of drugs during the delivery period should also consider whether there is any effect on the newborn baby to be born. Therefore, in the middle and late stages of pregnancy and maternal use of drugs during delivery should be cautious. 5. Note: The nervous system continues to differentiate and develop throughout pregnancy, so the effects of drugs on the nervous system persist. The U.S. Food and Drug Administration (FDA) classification of drug hazards to the fetus: Class A: safe for pregnant women, no harm to the embryo and fetus, such as moderate amounts of vitamin A, B1, B2, C, D, E, etc. Class B: safe for pregnant women, basically no harm to the fetus Class C: animal experiments on fetal teratogenicity, killing the embryo, not confirmed in human studies; therefore, pregnant women should only apply drugs when the benefits outweigh the risks. Grade D: There is evidence of harm to the fetus, so it should not be considered unless the pregnant woman has absolute effect after using the drug. Grade X: May cause fetal abnormalities and is prohibited during pregnancy.