The occurrence of malformations in the embryo after the action of teratogenic factors depends not only on the nature of the teratogenic factors and the genetic characteristics of the embryo, but also on the stage of development when the embryo is subjected to teratogenic factors. The first stage refers to 1 to 2 weeks (17 days) after fertilization (3 to 5 weeks after the last menstruation), when the embryo cells are totipotent and not yet differentiated, and the action of teratogenic factors at this time, if it is light, the embryo will survive normally, if it is heavy, the whole embryo or most of the cells can be damaged, resulting in embryo death or miscarriage; the second stage refers to the 3rd to 8 weeks (17 to 8 weeks) after fertilization, when the embryo is not yet differentiated. The second stage refers to the 3rd to 8th week (17-57 days) after fertilization (5-10 weeks after the last menstrual period), when the embryo cells are differentiated and highly sensitive to teratogenic factors, which is also called the embryo-sensitive stage and prone to teratogenicity. The third stage is from the beginning of the 9th week after fertilization (11 weeks after the last menstrual period) to delivery, when the fetal functions are mostly impaired due to the influence of adverse factors, which is less teratogenic. However, the central nervous system and sexual organs take a longer time to differentiate and develop, and they remain sensitive to teratogenic factors until the late pregnancy.