What is the primary site for nutrient exchange between mother and fetus?

The placenta is the primary site for nutrient exchange between the mother and fetus. It acts as an interface that facilitates the transfer of oxygen, nutrients, and waste products between maternal blood and fetal blood. This organ develops in the uterus during pregnancy and is essential for the growth and development of the fetus, providing everything it needs from the mother's circulation.

The chorion, while part of the placenta and involved in some aspects of fetal development, is not the main site for nutrient interchange. The amniotic sac primarily contains amniotic fluid, which cushions the fetus but does not play a direct role in nutrient exchange. The umbilical cord serves as a conduit for blood vessels that transport oxygen and nutrients to the fetus and return waste to the mother, but it does not itself facilitate the exchange; that function is fulfilled by the placenta.

Therefore, the placenta's unique structure, which includes villi that increase surface area for exchange and its intimate vascular relationship with maternal blood, makes it the correct answer regarding the primary involvement in nutrient transfer during pregnancy.