<P> The placenta functions as a fetomaternal organ with two components: the fetal placenta (Chorion frondosum), which develops from the same blastocyst that forms the fetus, and the maternal placenta (Decidua basalis), which develops from the maternal uterine tissue . It metabolises a number of substances and can release metabolic products into maternal or fetal circulations . </P> <P> Placental mammals, such as humans, have a chorioallantoic placenta that forms from the chorion and allantois . In humans, the placenta averages 22 cm (9 inch) in length and 2--2.5 cm (0.8--1 inch) in thickness, with the center being the thickest, and the edges being the thinnest . It typically weighs approximately 500 grams (just over 1 lb). It has a dark reddish - blue or crimson color . It connects to the fetus by an umbilical cord of approximately 55--60 cm (22--24 inch) in length, which contains two umbilical arteries and one umbilical vein . The umbilical cord inserts into the chorionic plate (has an eccentric attachment). 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 cotyledons . In humans, the placenta usually has a disc shape, but size varies vastly between different mammalian species . </P> <P> 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 multinucleated continuous cell layer that covers the surface of the placenta . It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development . The syncytiotrophoblast (otherwise known as syncytium), thereby contributes to the barrier function of the placenta . </P> <P> The placenta grows throughout pregnancy . Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy (approximately 12--13 weeks). </P>

Placenta develops from the outer cells of the