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Why do maternal and fetal blood supplies remain separate in the placenta?

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A series of medical illustrations showing the stages of childbirth: the baby's position at the onset of labor, flexion, internal rotation of the head, extension, external rotation of the head, and the uterus immediately after birth with the placenta and umbilical cord. — birth Sequential changes in the position of the child during labour.

In the placenta, maternal and fetal blood supplies remain separate to facilitate the efficient exchange of nutrients, oxygen, and waste products while preventing the direct mixing of the two circulatory systems.12

This separation is maintained by the placental barrier, which consists of the fetal blood vessel walls, the connective tissue of the villous core, and a layer of cells known as the syncytiotrophoblast. This structure allows for the diffusion of essential substances—such as oxygen, nutrients, and antibodies—from the mother’s blood into the fetal circulation, and the removal of carbon dioxide and nitrogenous wastes from the fetus into the maternal circulation.21

Maintaining this barrier is vital because direct mixing of maternal and fetal blood could lead to serious complications, such as erythroblastosis fetalis. This condition can occur if there is a blood group incompatibility (such as the Rh factor) between the mother and child, causing the mother's immune system to produce antibodies that cross the placenta and destroy fetal red blood cells.34