Drugs taken at different times of pregnancy have different effects on the fetus. Drugs during pregnancy can affect maternal endocrine and metabolism, which can indirectly affect the embryo and fetus, or directly affect the fetus through the placental barrier. The most serious drug toxicity can affect embryonic differentiation and development, leading to fetal malformations and dysfunction. Moreover, the side effects of drug administration are closely related to the timing of drug administration and gestational age. Generally, during the pre-fertilization period, which is the period between the fertilization of the egg and the endometrium of the fertilized egg, the fertilized egg is not in direct contact with the maternal tissues and is still in the fallopian tube lumen or uterine secretion, so the effect of medication on the fetus during this period is not significant. During the late blastocyst implantation to about 12 weeks of pregnancy is a sensitive period for drugs to cause fetal malformations. After 12 weeks of gestation until delivery, the organs of the fetus are already formed and the teratogenic effect of drugs will be significantly weakened.