When the fetuss trophoblast does not fully extend into the uterine wall, the spiral arteries do not become fully converted into low-resistance channels. It has been found that this incomplete conversion of spiral arteries increases the resistance to uterine blood flow during pregnancy, and that this occurrence was associated with gestational hypertension. One potential cause of this incomplete breach of the spiral arteries that leads to gestational hypertension is a mistaken immune response by the maternal tissue, reaction to the alien fetal tissue. Therefore, it is clear that the complication of gestational hypertension has roots in the early implantation of the fetus in the uterine wall, an implantation technique that is unique to humans.The highly invasive placenta that is found in humans is thought to be linked to humans high circulating levels of the hormones CG and hCG. It has been shown that the higher the levels of these hormones, the deeper the trophoblasts invasion into the uterine wall. Instances of gestational hypertension and pre-eclampsia have been shown to occur when the invasion of the uterine wall is not deep enough, because of lower CG and hCG levels in the mother. Evolutionary tradeoff
Despite these risks for gestational hypertension, the hemochorial placenta has been favored because of its advantages in the way that it aids in diffusion from mother to fetus later in pregnancy. The bipedal posture that has allowed humans to walk upright has also led to a reduced cardiac output, and it has been suggested that this is what necessitated humans aggressive early placental structures.
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.