The yolk sac is the precursor to the development of the fetal buds and the fetal heart, which usually appear first in pregnancy, followed by the development of the fetal buds and the fetal heart. The gestational sac is developed from the fertilized egg, and the gestational sac is the tissue of the primordial embryo. When the embryo is not formed, the gestational sac is present first, and then develops into the yolk sac, which is mostly suggestive that the embryo can develop, and then the fetal buds and the fetal heart will appear. The yolk sac can provide nutrients for the growth and development of the fetal buds, and the absence of yolk sac development can easily lead to the phenomenon of fetal arrest. Generally, the gestational sac and the yolk sac can be seen when the pregnancy is about 6-7 weeks old, and the development of the fetal buds and the fetal heart will occur at an interval of about 1 week. Regular checkups are needed during pregnancy to determine the development of the embryo.