hemochorial

Hemo-chorial placentation is a type of placentation in which maternal blood is in direct contact with the fetal chorion. In this configuration, most or all maternal tissue between the blood-filled uterine spaces and the chorionic trophoblast is removed or eroded during placental development, leaving the fetal trophoblast and its villous structures in direct contact with maternal blood.

Anatomically, the placental barrier in a hemochorial placenta is reduced to one or a few trophoblast cell layers, typically including syncytiotrophoblast and residual cytotrophoblast, separating fetal vessels from maternal blood. Maternal blood collects in intervillous spaces and bathes the chorionic villi, enabling efficient exchange of gases, nutrients, and wastes between mother and fetus. The arrangement can also influence immune interactions at the maternal–fetal interface.

Species with hemochorial placentation include humans and other primates, and many rodents; there is notable variation in how deeply the trophoblast invades maternal tissues and in the precise placental architecture across species. Hemochorial placentation is often associated with a relatively invasive trophoblast and a thinner placental barrier, contrasts with less invasive types such as endotheliochorial and epitheliochorial placentation, where more maternal tissue remains between blood and fetal tissue.

Overall, the term describes a direct exposure of fetal chorionic tissue to maternal blood, with implications for nutrient transfer, immunological interaction, and placental structure.