Humans Gestational hypertension is one of the most common disorders seen in human pregnancies. Though relatively benign on its own, in roughly half of the cases of gestational hypertension the disorder progresses into pre-eclampsia, a dangerous condition that can prove fatal to expectant mothers. However, gestational hypertension is a condition that is fairly rare to see in other animals. 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.
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 mother's blood. The hemochorial placenta thereby promotes more rapid diffusion to and from the fetal blood supply. In animals with an epitheliochorial placenta, 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 to grow the large brains compared to the body size that distinguishes 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. When the fetus's trophoblast does not fully extend into the uterine wall, the spiral arteries do not become fully converted into low-resistance channels. 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 unique to humans. The highly invasive placenta 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 trophoblast's 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 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 reduced cardiac output, and it has been suggested that this is what necessitated humans' aggressive early placental structures. Increased maternal blood pressure can attempt to make up for lower cardiac output, ensuring that the fetus's growing brain receives enough oxygen and nutrients. Gestational hypertension in the early stages of pregnancy (trimester 1) has been shown to improve the health of the child both in its first year of life, and its later life. However, when the disease develops later in the pregnancy (subsequent trimesters), or turns into pre-eclampsia, detrimental health effects for the fetus begin, including low birth-weight.
Relevance of evolutionary history It is the goal of
evolutionary medicine to find treatments for diseases that are informed by the evolutionary history of a disease. It has been suggested that gestational hypertension is linked to insulin resistance during pregnancy. Both the increase in blood sugar that can lead to gestational diabetes and the increase in blood pressure that can lead to gestational hypertension are mechanisms that aim to optimize the amount of nutrients that can be passed from maternal tissue to fetal tissue. It has been suggested that techniques used to combat insulin insensitivity might also prove beneficial to those with gestational hypertension. Measures to avoid insulin resistance include avoiding obesity before pregnancy, minimizing weight gain during pregnancy, eating foods with low glycemic indices, and exercising. ==References==