page_title: Gestational hypertension
section_title: Origins of the placenta
hierarchical_section_title: Gestational hypertension / Evolutionary considerations / Origins of the placenta
caption_attribution_description: Scheme of placental circulation.
context_page_description: Gestational hypertension or pregnancy-induced hypertension is the development of new hypertension in a pregnant woman after 20 weeks' gestation without the presence of protein in the urine or other signs of pre-eclampsia. Gestational hypertension is defined as having a blood pressure
  greater than 140/90 on two separate occasions at least 6 hours apart.
context_section_description: 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 mother's 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.  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 mother's blood that surround fetal tissue.  The lacunae are filled by the spiral arteries, which means that the mother's 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.
caption: Placental circulation
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