The chorion lies closest to the placenta. The chorionic sac encompasses the fetus and the amnion and is covered with chorionic villi until the 8th week. After the 8th week, these villi are ...
Chorionic villus sampling (CVS ... between 10 and 12 weeks of pregnancy. Because the placenta is derived from the extraembryonic portion of the early embryo, the villus cells in the placenta ...