The yolk sac is the earliest structure to appear in the gestational sac, usually at 5-6 weeks of pregnancy and no later than 8 weeks of pregnancy. The main structures of the yolk sac are the extra-embryonic endoderm and the extra-embryonic mesoderm, which are rich in nutrients and initially function mainly for blood production. As the embryo continues to develop, the yolk sac will gradually shrink and absorb. If the yolk sac does not appear for a long time, it may be due to poor development of the gestational sac or endocrine disorders, and it is recommended to visit a gynecologist for a timely examination. During pregnancy, pregnant women are advised to go to the hospital for regular maternity checkups to monitor the development of the embryo and fetus. Pregnant women in the early stage need to take adequate amount of folic acid and enhance nutrition to promote embryonic development. In addition, strenuous exercise and sexual life should be prohibited.