It is normal for the diameter of the gestational sac to be around 10.0mm at 40 days of pregnancy, and there may be some variation between individuals. For women who usually have regular menstrual cycles that are fixed between 28-30 days, the time of ovulation is relatively fixed. The 40th day of pregnancy is equivalent to about 24 days after ovulation. At this time, ultrasound examination can see a relatively obvious gestational sac, in which a soft yolk sac can be found, but no obvious embryonic buds or fetal heartbeat can be seen. Normally, the gestational sac at about 40 days of pregnancy is round or oval with thick walled cystic echoes. If the gestational sac is not visible in the uterine cavity on ultrasonography at around 40 days of pregnancy, is small in size, or has changed in shape, there may be an abnormal embryonic development.