At 37 days of menopause, the gestational sac is usually not visible under the ultrasound. This is because although the menopause is 37 days old, the fertilized egg has only been fertilized for about 23 days, because 14 days have to be subtracted from the previous one, which is the time for the follicles to develop. After fertilization, it takes about a week for the fertilized egg to implant in the uterine cavity and 4 to 5 days for the implantation to take place. When the egg has just implanted, it is still in the form of a cell, so the gestational sac cannot be seen on the ultrasound. In general, it takes at least 40 days of menopause to see the gestational sac, and it is better to do an ultrasound because the gestational sac is too small at this time, so it may be clearer to do an ultrasound.