What do you mean by yolk sac?

Yolk sac is usually seen in early pregnancy, through B-ultrasound examination can be seen in the uterus yolk sac situation, belongs to normal physiological phenomenon. If the examination results appear yolk sac, can be reexamined in a week, diagnosed whether it is yolk sac. If necessary, can be combined with blood HCG value and progesterone test results to judge, diagnosed embryo development is normal.