A 3mm yolk sac indicates an intrauterine pregnancy and the presence of an embryo. The yolk sac is the round sac-like structure that appears next to the germ and is the first structure observed within the gestational sac, which is strong evidence of pregnancy. A clear yolk sac is usually seen in the uterine cavity on ultrasound at 5 weeks of gestation, and it can be inferred that the embryo is well developed, and that the fetal buds and fetal heartbeat can be seen on repeat ultrasound at about 1 week of gestation. After seeing the yolk sac on ultrasonography, the size and shape of the yolk sac should also be observed, the normal diameter is 3~8mm, when the diameter of the yolk sac is >10mm or the yolk sac does not show, <3mm, denaturation or strong echogenicity within the yolk sac, and other changes are mostly indicative of a poor prognosis. Observation can be carried out under the guidance of a doctor, and the examination should be repeated on time.