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The placenta is an ephemeral organ present in placental vertebrates. The term placenta comes from the Latin for cake, from Greek plakoenta, accusative of plakoeis - p?a??e??, "flat"^[1][2], referring to its appearance in humans. Protherial (egg-laying) and metatherial (marsupial) mammals do not produce a placenta. The placenta develops from the same sperm and egg cells that form the fetus, and functions as a foetomaternal organ with two components, the foetal part (Chorion frondosum), and the maternal part (Decidua basalis).

In humans, the placenta averages 22 cm (8.66 in.) in length and 2-2.5 cm (0.79-0.98 in.) in thickness. It typically weighs approximately 500 grams (17.64 oz.). It has a dark reddish/blue or maroon colour. It connects to the foetus by an umbilical cord of approximately 55-60 cm (21.65-23.62 in.) in length that contains two arteries and one vein.^[3] The umbilical cord inserts into the chorionic plate. Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotelydons. In humans the placenta usually has a disc shape but different mammalian species have widely varying shapes.^[4]

The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium. The outer layer of the blastocyst becomes the trophoblast which forms the outer layer of the placenta. This outer layer is divided into two further layers; the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer. The syncytiotrophoblast is a multinucleate continuous cell layer which covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process which continues throughout placental development. The syncytiotrophoblast (otherwise known as syncytium), thereby contributes to the barrier function of the placenta.

The placenta grows throughout pregnancy. Development of the maternal blood supply to the placenta is suggested to be complete by the end of the first trimester of pregnancy (approximately 12-13 weeks).

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