Generally, the yolk sac can be seen under ultrasound at 5 weeks of pregnancy and the fetal heart and germ can be seen in the uterine cavity at about 6 weeks of pregnancy, so the yolk sac can be seen one week after the appearance of the fetal heart and germ, but there may be individual variability from patient to patient. The yolk sac and the germ are different stages of embryonic development. In women with regular menstrual cycles, i.e., about 28-30 days, their yolk sac can be seen on ultrasound at five weeks of postconception, gradually shrinking at 10-12 weeks, and the yolk sac, fetal bud and primordial heart tube pulsation can be seen simultaneously at six weeks of postconception. If no fetal bud or heartbeat is seen during the ultrasound examination at 6 weeks of pregnancy, it may be caused by the delayed ovulation day due to irregular menstruation of the patient and it is recommended to perform another ultrasound examination a week later to rule out embryonic abortion. In addition, it should be noted that if the yolk sac is deformed or has abnormal morphology, it indicates abnormal development. Therefore, it is recommended that pregnant women have regular ultrasound examinations in early pregnancy to help clarify the condition of the yolk sac and avoid miscarriage.