Schematic diagram of mature human placenta. The sagittal section of the uterus (left) with the developing embryo shows the most common location of the placenta. The mature placenta (right) is divided into cotyledons by placental septa that are formed by outgrowths of the decidua basalis. Maternal blood enters the placenta through numerous endometrial spiral arteries that penetrate the basal plate. As the blood enters the cotyledon, it is directed deep into the intervillous spaces (red arrows). It then passes over the surface of the villi, where umbilical vein chorionic plate amnion
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