Etymology
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placenta (n.)

1670s of plants, "part of the ovary of flowering plants which bears the ovules," 1690s of mammals, "organ of attachment of a vertebrate embryo or fetus to the wall of the uterus or womb of the female," from Modern Latin placenta uterina "uterine cake" (so called 16c. by Italian anatomist Realdo Colombo), from Latin placenta "a cake, flat cake," from Greek plakoenta, accusative of plakoeis "flat," from plax "flat, flat land, surface, plate," from PIE root *plak- (1) "to be flat." So called from the shape.

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placental (adj.)

"of or pertaining to a placenta," 1784, from Modern Latin placentalis, from placenta (see placenta).

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afterbirth (n.)
also after-birth, "placenta, etc., expelled from the uterus after birth," 1580s, perhaps based on older, similar Scandinavian compounds; see after + birth (n.). As "a birth after the death or last will of the father," 1875 (translating Latin agnatio in Roman law). Old English had æfterboren (adj.) "posthumous" in reference to birth.
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