The relationship between the fetal heart and the fetal yolk sac is the different periods of gestational sac development, which is what is shown under ultrasound. In case of a regular menstrual cycle, which is about 28 days, the yolk sac can be seen in the uterus by ultrasound at about 35 days of menopause, and at 6 weeks of menopause, the fetal bud and the primitive heart tube pulsation can be seen, it is the different periods of development. At 14 weeks of menopause it is possible to measure the head-rump length of the fetus by ultrasound. Measuring the head-rump length at this time enables a more accurate estimation of the gestational week and prevents the inaccuracy of the due date caused by too much time, when the due date can be corrected.