The yolk sac is usually between 3 and 6mm in size on the seventh and fifth day of pregnancy, and should have a clear morphology. The yolk sac is the small round sac-like structure next to the embryo bud in the pregnancy sac, in about 5 weeks of pregnancy ultrasonography shows that the yolk sac has a clear morphology, and most of them will be gradually shrinking and disappearing after 10 weeks of pregnancy, and the normal size of the yolk sac is between 3~6mm, so the size of the yolk sac on the seventh and fifth days of pregnancy is normal in this range, and the morphology is also clear and full. When the ultrasound shows a normal size and clear yolk sac, and there is no fetal heartbeat temporarily, it can also be inferred that the embryo is well developed. However, if there is no fetal heartbeat when the yolk sac is ≥6.1mm, the embryo may be underdeveloped, and the diagnosis should be supported by progesterone, hCG, and estradiol, and progesterone and other medications should be given as prescribed by the doctor.