The main difference between a blastocyst and a gestational sac is that a blastocyst is a blackberry embryo that is formed when the fertilized egg begins to undergo mitosis, and this is the early blastocyst. The blastocyst that enters the uterine cavity and is ready for implantation is known as the late blastocyst, and the late blastocyst that implanted in the uterine cavity and began to grow and develop is what forms the gestational sac. Most of the blastocysts cannot be seen by ultrasound, but only the gestational sac can be seen by ultrasound. If you have a regular menstrual cycle of 30 days, you can see the gestational sac by ultrasound only after 35 days of menopause, and the corresponding yolk sac can be seen inside the gestational sac at around 40 days.