Uteroplacental blood flow. The story of decidualization, menstruation, and trophoblast invasion.

Sexual reproduction in the ocean necessitates only the combination of gametes, followed by absorption of nutrients and oxygen from the surrounding watery medium. As life moved from the sea to the land, reproductive strategies required compensation for the loss of this aquatic environment. For mammals and a few other animals, the solution to this problem was the development of the placenta, the means by which the fetus extracts nutrients from its environment. As the animals that used the placenta evolved from small rodent-like creatures with short gestations to larger animals with prolonged gestations, the demands of the developing fetus grew. Whereas the placenta of the fetal pig, with a gestational period of a little less than 4 months, can extract sufficient nutrients from the mother by simple diffusion across the uterus to the placenta, the human fetus needs a far more complex uteroplacental relationship. Several evolutionary solutions to the increased demands of fetuses can be observed. One approach was a larger placenta. For example, the chinchilla has a neonatal:placental weight ratio of 30:1, whereas the human has a 6:1 ratio. Another means to greater nutritional support for the fetus was to increase the surface area of contact between fetal circulation in the placenta and maternal circulation. The pig fetus has a diffuse placenta that makes contact with the mother’s uterus by a simple folded contact. The human placenta, on the other hand, has a complex villous structure, similar to the sea anemone’s tentacles waving in the sea, that greatly increases the contact surface area between the mother’s blood space and the fetal circulation. Despite this increased fetal-maternal contact, the system is still rather inefficient. We can quantify this by considering the amount of oxygen in the maternal blood that enters the human placenta and the amount of oxygen in the fetal blood that leaves the placenta. Maternal blood has a pO2 of around 100, whereas the pO2 of umbilical vein blood is around 35 to 40. This represents an efficiency of only 35 to 40%. Therefore, it also became necessary to greatly increase the flow of maternal blood into the intervillous space during pregnancy. Without this increased maternal blood flow, preterm birth and fetal loss occur. One of two mechanisms can increase maternal flow: increased total body blood flow or increased blood flow to the placental bed through the uterine spiral arteries. For the human, evolution has selected the latter mechanism, limiting the overall systemic effects that increased total body blood flow would produce.