The chorionic villi and decidua basalis form the _____.

• A. Placenta
• B. Amnion
• C. Yolk sac
• D. Chorion

Answer: (A)

Placentation is the formation of the placenta, the organ that enables nutrient, gas exchange, and waste removal between mother and fetus. The fetal portion comes from the chorion, specifically the chorionic villi, which grow into the maternal lining. The maternal portion is the decidua basalis, the part of the endometrium that has transformed during pregnancy. When the chorionic villi invade the decidua basalis, their interfacing tissue forms the placental structure, with fetal vessels inside the villi and maternal blood in the surrounding spaces to allow exchange. The amnion is the protective membrane around the fetus, the yolk sac provides early nourishment and hematopoiesis, and the chorion is the outer fetal membrane—these are not the placenta themselves. Therefore, the combination of chorionic villi and decidua basalis forms the placenta.