Pregnancy can be seen on ultrasound at around 40 days. Most women have a very regular menstrual cycle of 28-30 days and ovulation is relatively regular, occurring in the middle of two periods. For example, if you have your period on January 1, you will ovulate around the 15th, and if you conceive at this time, your urine and blood pregnancy test will be positive, but the fertilized egg is not in bed at this time, which means that the fertilized egg is on its way, so the ultrasound will not be able to show it, and you will need to do an ultrasound on February 10th, after the fertilized egg has taken root in its place, to be able to see the gestational sac. However, for the few women who have irregular periods and ovulation, an ultrasound is recommended to monitor the development of the follicles. When the diameter of the follicle reaches 18-25mm, you can have intercourse, and at this time, conception can be projected back about 25 days to do ultrasound, and you can usually see the gestational sac.