The effect of medication on the fetus within 35 days of pregnancy needs to be determined by the class of medication and the specific time of pregnancy when the medication is used. The U.S. FDA has classified drugs into 5 levels: A, B, C, D, X, based on their teratogenic effects on the embryo and fetus; A level is non-teratogenic drugs, such as moderate amount of vitamins; B level can be used under the observation of physicians, such as penicillin, erythromycin, digoxin, insulin, etc.; C level may be teratogenic to the fetus and should be used as an option on the balance of pros and cons. The use of these drugs is prohibited during pregnancy. During the pre-fertilization period, i.e. 2 weeks after fertilization, the effect of drugs on the embryo is completely inactive. The teratogenic period of drugs is from the post-fertilization period to 12 weeks, so it is not suitable to apply C, D and X grade drugs; after 12 weeks, the teratogenic effect of drugs decreases, but the effect on reproduction and nervous system still exists. It should be clarified whether the medication is to be taken within 35 days of pregnancy or between 28 days and 35 days, and the type of medication used also has an effect. So it is also necessary to clarify the type of medication to be administered. After pregnancy, drugs can affect the embryo and fetus indirectly by affecting the maternal endocrine and metabolism, or directly through the placenta, and the most serious drug toxicity is affecting the differentiation and development of the embryo. Preimplantation is the period between fertilization of the egg and implantation of the fertilized egg in the endometrium, which refers to the 2 weeks after fertilization. At this stage, the fertilized egg is not in contact with the mother, so the drug has little effect on it; the late blastocyst period after implantation to about 12 weeks is the teratogenic period of the drug, which is a critical period for the embryo and fetal organs to be highly differentiated, rapidly developed and continuously formed, with the heart and brain being the first to be formed, followed by the eyes and limbs. The earlier the toxic effect of drugs, the more serious the possibility of malformation. After 12 weeks of gestation to delivery, all organs of the fetus have been formed and the teratogenic effect of drugs is obviously weakened; however, some drugs can still affect the organs that have not yet been differentiated, such as the reproductive system, and the nervous system continues to develop throughout pregnancy, so the effect of drugs on it is always present. In conclusion, it is important to be careful with medications during pregnancy and to consult an obstetrician before using any medication.