Level 1: Changes in the size of the gestational sac at 40 days of pregnancy. At 40 days of pregnancy, the size and shape of the gestational sac will change. At this time, the gestational sac is generally a small round structure with a diameter of about 4-6 mm, which can be clearly observed during ultrasound examination. As the embryo develops, the gestational sac gradually increases in size, reaching its maximum size at 8-10 weeks. Level 2: embryo development at 40 days of pregnancy. At 40 days of pregnancy, the embryo