The gestational sac usually consists of three data instead of two. The three data of the gestational sac represent the length, width and height of the gestational sac, i.e., the long, right and left, and anterior and posterior diameters of the gestational sac. Based on the three data of the gestational sac, the average internal diameter of the gestational sac and the gestational week can be calculated by using the formula. The gestational sac can also become the fetal sac, which serves as the original placental tissue, and its size can be observed to see if the fetus is growing normally. The normal location of the gestational sac can be in the upper, middle, anterior, posterior, or lateral wall of the uterus, and its shape is more regular. For pregnant women with irregular periods, it is more difficult to calculate the week of pregnancy by projecting the ovulation period, but the size of the gestational sac can be used to calculate the week of pregnancy. When pregnant women undergo ultrasound, the physician can know the exact size of the gestational sac by measuring its long diameter, right and left diameter, and anterior and posterior diameter. Based on the three data of the gestational sac, the average internal diameter of the gestational sac is derived by using the relevant formula, i.e. (long diameter + left and right diameter + anterior and posterior diameter)/3cm, and accordingly, the gestational week is calculated by using the formula of (average internal diameter + 2.543)/0.7. Pregnant women can check the size of the gestational sac and observe the growth of the fetus by means of regular ultrasound examinations. Once an abnormal size of the gestational sac is detected, a physician should be contacted in time to make a clear diagnosis and carry out targeted treatment at the earliest possible time.