Placenta and Embryo at About 8 Weeks
At 8 weeks of pregnancy (6 weeks after fertilization), development of most major organ systems has begun in the embryo. The placenta has also developed and formed tiny finger-like projections (villi) that extend into the wall of the uterus.
The villi are part of the circulatory system of the embryo. Blood vessels carry blood from the embryo through the umbilical cord and the placental villi. Then the blood returns to the embryo. Blood vessels from the mother pass next to the placental villi, and maternal blood fills the space around the villi. The blood vessels of the mother and the embryo are separated by a thin membrane. Blood does not run directly from the mother to embryo.
Fluid, oxygen, and nutrients pass across the membrane from the mother to the embryo, and carbon dioxide and waste products pass from the embryo to the mother.
Cells from the placenta also develop into the amniotic sac. Two layers of membranes form around the embryo: the amnion (inner membrane) and chorion (outer membrane). The amnion and chorion create a sac (amniotic sac) around the embryo. The sac is filled with fluid (amniotic fluid) and the embryo floats in the fluid.
The amniotic fluid provides a space in which the embryo can grow freely and helps protect the embryo from injury. The amniotic sac is strong and resilient.