What are Chorionic Villi?

Chorionic villi are villi that sprout from the chorion in order to give a maximum area of contact with the maternal blood.

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.

Since both the chorionic villi and the embryo are derived from the same fertilized ovum, a sample of the former provides material for genetic studies of the latter. Chorionic villus sampling has become an important method of early pre-natal screening for genetic defects.