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Placental development timeline, trophoblast invasion, and mature placental structure. (a) Diagram of trophoblast invasion around day 9, wherein the syncytiotrophoblast layer surrounding the embryoblast begins to invade the endometrium. (b) Mature placental structure showing maternal and fetal vasculature. (c) Focus on exchange of nutrients between open maternal blood and closed fetal circulation across the two trophoblastic layers.
