The yolk sac is usually 3-8mm normal. The yolk sac provides nutrition, metabolism, immunity, and hematopoietic functions for the initial embryonic development. Usually in early pregnancy, the yolk sac can be seen in the uterine cavity by ultrasound, indicating that the fetus is better developed. In general, the normal value of the yolk sac is between 3-8mm. If the yolk sac is obviously larger than 8mm, it is on the large side, and is prone to fetal malformation or miscarriage. It is normal for the yolk sac to shrink and disappear as the pregnancy progresses. At around 42 days of pregnancy, it is recommended to go to the obstetrics and gynecology department of a regular hospital to have an ultrasound examination to observe the size of the yolk sac and learn about the development of the embryo in time. Once the yolk sac is found to be abnormally large, we should pay attention to it and cooperate with the doctor to complete the relevant examinations as soon as possible, so as to avoid triggering a miscarriage, and if necessary, take measures to preserve the fetus.