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Histological specimens of recent implantation sites are the basis of our current concept on human embryo implantation and placental development. In the Carnegie Collection maternal red blood cells were detected early in the primitive intervillous space (10th-12th day after conception). These cells were localized to the trophoblastic lacunae and originated from distended peripheral maternal sinusoids (Kaufmann, 1981). The classical theory states that progressively more and more maternal vessels are tapped. A true maternal blood flow is established around the 29th day. Dynamic investigations of human placental development in vivo are scarce and hampered by ethical considerations and the absolute requirement to refrain from using non aggressive and potentially harmful techniques. Despite these limitations such studies provide new insights that surprisingly contradict our previously and seemingly definitely established knowledge of the early phases of placental vascularization, and lead us to conclude that there is an absence of maternal blood circulation in the intervillous placental space (IVS) during the 12 first weeks of human pregnancy.

The human placenta becomes haemochorial at the 13th week of pregnancy.