To calculate the diameter of the gestational sac, it is usually necessary to measure the length, width and thickness of the gestational sac, add these three values and multiply the total by three. The diameter of the gestational sac is usually the average diameter of the gestational sac, and the size of the diameter of the gestational sac can be used to make a preliminary assessment of the number of days of pregnancy, especially in pregnant women who have irregular periods and forget their last menstrual period. Measurement of the diameter of the gestational sac can be performed by abdominal ultrasound or vaginal ultrasound. Vaginal ultrasound is relatively accurate in measuring the diameter of the gestational sac, which can be measured from the medial side of the gestational sac to the medial side of the opposite rim, and accurately measures the width, length, and thickness of the gestational sac. If the gestational sac is small, it may be considered a pathologic condition, which needs to be evaluated based on clinical symptoms and hormone levels in the blood. Therefore, it is advisable to consult the obstetrics department of a regular hospital if the size of the gestational sac is found to be abnormal in clinical practice.