The yolk sac is the site of embryonic blood production and is usually formed around 3 weeks of gestation. If the yolk sac can be seen next to the embryo in early pregnancy, the fetus is considered to be well developed. If the gestational week increases and only the yolk sac is visible without the yolk sac or the fetal heart and germ, the fetus is considered to be poorly developed and may be subject to miscarriage or embryonic abortion. As the gestational week grows, the yolk sac slowly becomes smaller and disappears at about 12 weeks of gestation. The yolk sac is a sign of normal embryo development and may be detected by ultrasound at 6 weeks of pregnancy. If there is embryonic dysplasia, embryonic abortion or other conditions, it is recommended to go to the hospital for timely examination and treatment. At the same time, it is necessary to have regular maternity checkups and improve the macroscopic examination.