The gestational week is calculated from the first day of the last menstrual period, usually 2 weeks before ovulation or fertilization and 3 weeks before the time of implantation. The full course of pregnancy is approximately 280 days, or 40 weeks, and the human embryo within 10 weeks of gestation (8 weeks after fertilization) is called an embryo. At this time, the embryo has taken on a human form, with a large head that accounts for nearly half of the entire fetal body. The eyes, ears, nose, mouth, fingers and toes can be distinguished, organs are differentiating and developing, and the heart has been formed. The developmental process of the embryo is detailed and complex, and is a process of differentiation of cells and tissues in a certain order, and the embryonic stage is an important time for organ differentiation and formation, when all major external and internal structures begin to develop. Disturbance in any part of this process can lead to various malformations. In particular, organs are most vulnerable to disturbance by teratogenic factors when they are rapidly differentiating and developing. According to the study of human embryonic development time, the most dangerous period for malformation of major organs is during this period, such as 15-27 days after conception for brain, 24-29 days for eye, 20-29 days for heart, 24-36 days for limbs and 26-62 days for genitalia, which is called sensitive period. When the embryo enters the fetal stage, the organs and tissues gradually develop and take shape, and then the sensitivity to harmful substances decreases, and if they are attacked, they mostly show some physiological dysfunction or small physical abnormalities.