The human placenta is a hemochorial, villous type where the maternal blood enters the intervillous space via the spiral arteries and flows directly around the terminal villi of the fetal circulation without any intervening maternal vessel wall. The surface area available for exchange gradually increases throughout pregnancy until it reaches around 10-15 m2 in the last trimester (Figure 1). The nature of the exchangeable surface of the placenta also changes throughout gestation with the mature intermediate villi appearing towards the end of the second trimester and the terminal villi, which represent the main site of feto-maternal exchange, appearing a few weeks later. The rate of fetal blood delivery to the placenta (umbilical flow) also changes markedly during pregnancy and is approximately linearly related to fetal weight, and hence the fetal nutrient requirement, throughout gestation (Figure 1).
Anatomically, the human placenta is a large structure typically weighing around half a kilogram. However, its physical bulk belies the flimsy nature of the separation between the maternal and fetal
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