At 8 weeks of pregnancy, the placenta and fetus have been developing for 6 weeks. The placenta forms tiny hairlike projections (villi) that extend into the wall of the uterus. Blood vessels from the embryo, which pass through the umbilical cord to the placenta, develop in the villi. A thin membrane separates the embryo's blood in the villi from the mother's blood that flows through the space surrounding the villi (intervillous space). This arrangement does the following: Allows materials to be exchanged between the blood of the mother and that of the embryo Prevents the mother's immune system from attacking the embryo because the mother's antibodies are too large to pass through the membrane The embryo floats in fluid (amniotic fluid), which is contained in a sac (amniotic sac). The amniotic fluid does the following: Provides a space in which the embryo can grow freely Helps protect the embryo from injury The amniotic sac is strong and resilient.