Chorionic villi are villi that sprout from the chorion in order to give a maximum area of contact with the maternal blood.
They are the sine qua non of pregnancy, from a histomorphologic perspective, and are by definition products of conception.
Embryonic blood is carried to the villi by the branches of the umbilical arteries, and after circulating through the capillaries of the villi, is returned to the embryo by the umbilical veins.
Thus, the villi are part of the border between maternal and fetal blood during pregnancy.
Read more about Chorionic Villi: Use For Prenatal Diagnosis, Development, Tissue Composition and Cell Types, Additional Images, Stem Cell