The yolk sac is the main structure in the gestational sac, and the difference between the two is in the time of formation. The gestational sac is the original embryonic tissue, after the sperm and egg combine to form a fertilized egg, it slowly splits to become a sac-like structure. At the beginning, the gestational sac is composed of placental tissue, blood vessel network, and amniotic membrane, etc. The gestational sac is the initial form of the fetus, which is not yet formed at this time. The yolk sac is formed during the gradual development of the embryo. The yolk sac is the earliest part of the embryo where blood vessels and blood cells are formed, and it is also the hematopoietic site of the early embryo. During embryonic development, the gestational sac gradually grows out of the yolk sac, which in turn provides nutrients to the embryo, without which the embryo cannot develop into a fetus. The yolk sac and the gestational sac are specific images presented by ultrasound at different times after pregnancy. In case of any abnormality, prompt medical attention is required.