Unseen yolk sac means that the yolk sac is not seen in the uterine cavity on ultrasound, which may be due to short pregnancy or poor embryonic development. The yolk sac is a round, sac-like structure next to the embryonic bud, which is usually visible on ultrasound at 5 weeks of gestation and gradually shrinks and disappears after 10 weeks of gestation. When the yolk sac is clear and of normal size on ultrasound, it indicates that the embryo is well developed. When the yolk sac is not visible on ultrasound, it may be due to the short duration of pregnancy, etc. After checking the gestational week, it can be observed for a while and be re-checked on time. When the number of days of pregnancy is sufficient after checking the gestational week, but the yolk sac is still not seen, there may be the possibility of poor embryonic development, but also combined with the progesterone and blood HCG (human chorionic gonadotropin) levels, if necessary, we need to follow the doctor’s advice to give progesterone, low molecular heparin and other drugs to protect the fetus treatment, and if there is no improvement in the review of the treatment, we need to terminate the pregnancy in a timely manner. When the yolk sac is not seen by ultrasound examination, the cause can be identified under the guidance of the doctor, and timely treatment.