The degree of drug effects on the fetus varies during different stages of fetal development. Within 2 weeks after fertilization, the effect of drugs on the embryo is all; or none; all i.e. embryonic death; none; i.e. the embryo is not affected. If most of the cells are damaged, the embryo dies, while a small number of cells die and the embryo continues to survive through compensatory mechanisms. The fetus is most sensitive to drugs during 3-8 weeks after conception (i.e. 5-10 weeks after menopause), when the embryo and fetal organs are in a highly differentiated, rapidly developing and continuously forming stage, and the possibility of structural abnormalities by harmful drugs is the greatest. After 9 weeks of fertilization, all organs of the fetus have been formed and their sensitivity to teratogenic drugs is obviously weakened and will not cause obvious malformations, but the reproductive system is not yet fully differentiated and may still be affected to different degrees. The neurological system continues to differentiate and develop throughout pregnancy, so the effect of drugs on it can always exist. Drugs used during delivery also have an effect on the fetus or newborn. In conclusion, the adverse effects of drugs on the fetus and infant are mainly related to the nature of the drug, the speed and dose of the drug reaching the fetus, the affinity of the fetus and the gestational age at the time of exposure to the drug. The basic principles of drug use during pregnancy】 Strictly control the indications for drug use and avoid unnecessary drug use. When a pregnant woman is ill and must use drugs, she should choose drugs that have little effect on the fetus under the guidance of a doctor, and drugs that have an effect on the fetus should be used after weighing the pros and cons. If the condition of early pregnancy allows, postpone the use of drugs until the middle or late pregnancy as far as possible. The U.S. Food and Drug Administration (FDA) classifies drugs into five hazard classes according to their risk to the fetus: Class A: In human studies with a control group, they have been shown to be harmless to the fetus. Such drugs are rare, such as moderate amounts of vitamins. Class B: Animal studies suggest no hazard to the fetus, but human studies are lacking; or adverse effects in animals, but no adverse effects on the fetus in well-controlled human studies, such as penicillin. Grade C: Lack of adequate studies in animals and humans, or adverse effects on the embryo in animal studies but lack of controlled studies in humans; many drugs commonly used during pregnancy fall into this category and their use must be carefully weighed against the effects of the drug on the fetus. Class D: There is evidence of embryonic harm, but there is an absolute benefit to the pregnant woman with the use of the drug, and it may be considered if the pregnant woman has a serious illness or is threatened with death in urgent need of the drug, such as aminoglutethimide and phenytoin sodium. Class X: Definite evidence of risk to the embryo and is contraindicated in women who are or will be pregnant. Examples include the acne treatment drug isotretinoin. In pregnancy, use Class A or B drugs when possible, use Class C drugs with caution, use Class D drugs on balance, and prohibit Class X drugs. Avoid combining drugs if you can use them alone, and avoid using new drugs that have not yet been determined to have adverse effects on the fetus if you can use drugs with more definite conclusions. Strictly control the dose and time of medication and stop it in time.