If ovulation is not advanced or delayed on the 40th day of pregnancy, the gestational sac can be seen under the ultrasound, but the yolk sac and embryo bud pulsation cannot be seen yet, as the embryo is still relatively small at this time, and there is no yolk sac produced yet, which refers to the normal time of ovulation. If the patient’s ovulation is delayed, the yolk sac may not be visible under the ultrasound, and a blood test is needed to check the value of HCG in the blood. If the ovulation period is advanced and the embryo is larger on ultrasound, the yolk sac may be found, which is a normal phenomenon.