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What are the chorion frondosum and decidua basalis?

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Close-up of a home pregnancy test device showing a blue plus sign in the result window, indicating a positive result for pregnancy. — pregnancy Devices for individual use at home detect pregnancy by measuring levels of the hormone human chorionic gonadotropin (hCG) in urine.

The chorion frondosum and decidua basalis are the two primary components that form the mature, disk-shaped placenta.1

Chorion frondosum: This is the fetal component of the placenta. It develops from the chorion—the outer membrane surrounding the fetus—specifically in the area where chorionic villi grow larger and more numerous on the side of the conceptus adjacent to the uterine wall.1
Decidua basalis: This is the maternal component of the placenta. It consists of the layer of the endometrium (the uterine lining) that lies directly beneath the encroaching conceptus. It integrates with remnants of the invading trophoblast and is eventually cast off when the placenta is expelled after birth.1

Together, these structures facilitate the exchange of nutrients, oxygen, and waste products between the maternal and fetal bloodstreams through the intervillous space.12