The yolk sac is usually visible at about five weeks of gestation. The yolk sac can be seen on ultrasound when a woman has regular menstruation, which is about one week after menopause. The diameter of the yolk sac is usually around 3-6mm. An oversized yolk sac may indicate abnormal fetal development and a high chance of fetal abortion. Usually when the HCG reaches about 8000 IU/mL, the yolk sac should be shown under the ultrasound. If the yolk sac is not visible, the risk of fetal abortion is suspected. Usually women without a history of recurrent miscarriage do not need to be observed for the presence of yolk sac, it is always enough to see if the ultrasound is intrauterine pregnancy and if there is a fetal heartbeat about 2-3 weeks after the pregnancy test. Patients with recurrent miscarriage usually have to observe the yolk sac to see if the fetus is developing normally. If the yolk sac is too large, there is a high risk of fetal abortion and the medication should be stopped in time.