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Proceedings of the British Institute of Radiology

The human placenta is a discoid extra-corporeal organ of the fetus, with fetal and maternal surfaces. Approximately 60 main stem villi arise from the chorionic plate just below the fetal surface. Each of these subdivides into second order villous stems, which in turn subdivide into third order villous stems. The terminal branches of these are known as the anchoring villi and attach the maternal surface of the placenta to the decidua. It is from the surface of all the subdivisions of the main stem villi that an extensive system of terminal villi arises. Ramifying throughoutthe villous tree is a corresponding branching network of arteries, originating in the paired umbilical arteries and terminating in the capillaries within the terminal villi. The capillaries of the terminal villi drain into a corresponding system of branching venules, which ultimately converge on the umbilical vein. The extensive micro-circulation of fetal blood within the terminal villi is closely related to the maternal circulation of blood through the chorio-decidual or intervillous space. This maternal circulation originates from an estimated 100–400 spiral arteries (terminal branches of the uterine arterial system) which open at random on the basal plate. Venous drainage from the chorio-decidual space is via a similar number of randomly situated veins.