The gestational sac in the seventh week of pregnancy is usually about 2cm in diameter, and a fetal heart and germ will appear. The fetal heart and germ are usually about 0.9 cm, but this is for women who have regular menstruation. If ovulation is delayed, i.e., if menstruation is postponed, there is a possibility of late emergence of the fetal sac and fetal heart and bud. In the case of regular menstruation, the fetal sac can be seen on day 35 and the fetal bud usually appears around day 46 or 47. For women with irregular periods, the appearance may be delayed. Therefore, in most pregnant women with regular menstruation, the diameter of the sac is about 2 cm at the seventh month of menopause. The size of the sac under the ultrasound can also be used to infer the regularity of the patient’s menstruation and to check the expected date of delivery.