The yolk sac disappears around the 8th week and becomes the placenta, which can be fully formed around the 12th week of pregnancy. The yolk sac is the place where the embryo makes blood, and can usually be seen by ultrasound at 5-6 weeks of pregnancy. The yolk sac disappears around week 8, when the placenta gradually develops and takes over the role of the yolk sac. The yolk sac is the basis for determining normal intrauterine pregnancy in the early stages of pregnancy, is the embryo in the uterus after one week of development by the proliferation of embryonic cells formed, with the continuous development and growth of the embryo yolk sac gradually become smaller, forming a diameter of 5mm or so of the vesicles remaining on the surface of the embryo. After 12 weeks of pregnancy, the placenta is fully formed, and the various hormones needed for pregnancy will be secreted by the placenta, and the development of the fetus in the uterus will be more stable. The placenta is an important organ for material exchange between the fetus and the mother. The fetus draws nutrition from the mother through the placenta, thus ensuring normal growth and development of the fetus. It is important to have a good pregnancy checkup to know if the embryo is growing and developing normally.