Theoretically, the gestational sac (GS) is visible on ultrasound (vaginal ultrasound) at 5 weeks of gestation: a round or oval halo with clear borders is visible in the enlarged uterine cavity, which is an echogenic zone, and if the yolk sac or germ and rhythmic cardiotubular pulsations can be seen within it, the diagnosis of early pregnancy and live fetus can be confirmed. Pseudogestational sacs are formed when a woman has an abnormal pregnancy (ectopic pregnancy or ectopic pregnancy) and are surrounded by retained blood or mucus by the metastatic endometrium, which is a response of the endometrial glands and epithelial tissue to HCG. An empty gestational sac is an empty blastocyst >25 mm in diameter in the uterus, with no inner germ tube pulsation detected by ultrasound or only an empty blastocyst without embryonic tissue. How to identify the early gestational sac, false gestational sac and empty gestational sac by ultrasound? True gestational sac The gestational sac is usually located in the meconium on one side of the uterine cavity, with a smooth and complete outline and good tension, which basically corresponds to the gestational week; the wall thickness of the sac is usually >2mm, with uniform and strong echogenicity; it can show the double ring sign; the yolk sac and embryonic echogenicity can be seen in the gestational sac, and the primitive heart tube pulsation can be seen, and the yolk sac is the gold standard for the diagnosis of gestational sac; the true gestational sac can grow with the gestational age. The empty gestational sac is irregular in outline or defective, often smaller than the gestational age with a hollow shape and poor tension; the empty gestational sac may be located in the center of the uterine cavity or slightly to one side due to the death and withering of the embryo, and generally moves down to the cervix or the lower part of the uterine cavity; the empty gestational sac does not grow with the gestational age or grows slowly. The pseudogestational sac is irregular in outline and does not correspond to the gestational age, with a thin walled single ring-like dark area; the pseudogestational sac is usually located in the middle of the uterine cavity; the pseudogestational sac does not grow with gestational age or is reduced in size; a gestational sac or mass may appear in the adnexal region; in the tissue scraped out by intrauterine curettage, no embryonic tissue such as chorionic villi is seen, only meconium tissue is seen.