The high vascular resistance outside of the placenta leads to higher maternal blood pressure throughout the body.The fetal cells that implant into the uterine wall are known as the trophoblast. The hemochorial placenta bathes the fetal trophoblast in maternal blood by forming lacunae, or lakes, of the mothers blood that surround fetal tissue. The lacunae are filled by the spiral arteries, which means that the mothers blood pressure is the driving force behind the introduction of new blood, which contains both oxygen and food for the fetus, to the system. It is thought that humans need the increased diffusion provided by the hemochorial placenta in order to grow the large brains compared to their body size that distinguish them from other primates. Incorrect placental implantation
It is thought that "failings" in normal hemochorial placental structure lead to pre-eclampsia and gestational hypertension. The human placenta implants "earlier, deeper, and more extensively" into the uterine wall, which can potentially lead to many problems that are found in human pregnancies, but not as much in other animals. Miscarriage and pre-eclampsia are both very rare in other species, but are two of the most common pregnancy-related diseases in humans. The genetic roots of gestational hypertension and pre-eclampsia are certain, as women with a family history of the condition are three times more likely to develop it when they are pregnant.One of the potential causes of gestational hypertension and pre-eclampsia is when the trophoblast does not invade far enough into the uterine lining.
For years, it has been the belief of the scientific community that gestational hypertension and pre-eclampsia were relatively unique to humans, although there has been some recent evidence that other primates can also develop similar conditions, albeit due to different underlying mechanisms. The underlying cause of gestational hypertension in humans is commonly believed to be an improperly implanted placenta. Humans have evolved to have a very invasive placenta to facilitate better oxygen transfer from the mother to the fetus, to support the growth of its large brain. Origins of the placenta
The origins of gestational hypertension may lie with the development of humans hemochorial placenta. A hemochorial placenta optimizes the amount of oxygen and nutrients that can be absorbed into the fetal blood supply, while at the same time ensuring rapid diffusion of wastes away from the fetus. This hemochorial placenta differs from lower primates epitheliochorial placentae in the way that it allows the fetal tissues to interact directly with the mothers blood. The hemochorial placenta thereby promotes more rapid diffusion to and from the fetal blood supply.In animals with epitheliochorial placentae such as horses and pigs, the greatest resistance to maternal blood flow in the vascular system was found within the placenta. However, in animals with hemochorial placental structures such as rodents and primates, the vascular resistance in the placenta was low, leading scientists to the conclusion that the greatest resistance to maternal blood flow is found elsewhere in the maternal vascular system.