The placenta attaches to the uterine wall and allows metabolic exchange between the fetus and the mother. The placenta has both embryonic and maternal components. The embryonic portion comes from the outermost embryonic membrane. The maternal portion develops from the decidua basalis of the uterus. The placental membrane separates the embryonic blood from maternal blood but is thin enough to allow diffusion and transport of nutrients and waste. A normal placenta is round or oval-shaped and about 22 cm in diameter. It is 2 cm to 2.5 cm thick and weighs about a pound.
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