The gestational sac is the primitive placental tissue, a small embryo wrapped by the amniotic membrane and vascular network. As the embryo grows gradually, the germ and fetal heart are formed in the gestational sac. If the gestational sac, fetal heart and fetal bud are visible under ultrasound, intrauterine pregnancy is diagnosed. At 50 days of pregnancy, which is almost 7 weeks, the fetus grows to 1.33 cm, the embryo has a human prototype, all the body segments have been differentiated, the limbs are separated and the systems are further developed. The value of the gestational sac is not too big, so you can add up the three values of the gestational sac in millimeters from the ultrasound results, take the average and add 25 to get a value similar to 50. For example, if the value of the gestational sac is 20mm*20mm*30mm, if the ultrasound is done with two numbers, add the millimeters of these two values, take their average and then add 30, the value should be about the same as 50. Because the value of the gestational sac is composed of two or three groups of numbers, it is not a fixed number. Also, the standard size of a gestational sac in early pregnancy will change over time and each stage will have a standard size for each stage. In general, the standard size of a gestational sac grows and changes from 0.2cm in early pregnancy to about 5cm at the end. Therefore, pregnant women should go to the hospital regularly for ultrasound tests to keep observing the changes in the size of the gestational sac and to keep abreast of their situation.