Obstetrical dilemma

The term, obstetrical dilemma, was coined in 1960, by Sherwood Larned Washburn, a prominent early American physical anthropologist, in order to describe the evolutionary development of the human pelvis and its relation to childbirth and pregnancy in hominids and non-human primates. In the intervening decades, the term has been used broadly among anthropologists, biologists, and other scientists to describe aspects of this hypothesis and related topics. ==Evolution of human birth==

Human pelvis The obstetrical dilemma hypothesizes that when hominids began to develop bipedal locomotion, the conflict between these two opposing evolutionary pressures became greatly exacerbated. Because humans are currently the only recognized extant obligately bipedal primates, meaning the body shape requires to only use two legs, major evolutionary developments had to occur in order to alter the shape of the female pelvis. Human pelves have no gross distinguishing skeletal markers for sex before puberty. With puberty, hormones alter the shape of the pelvis in females to cater to obstetrical demands. Overall, through the evolution of the species, a number of structures in the body have changed size, proportion, or location in order to accommodate bipedal locomotion and allow a person to stand upright and face forward. To help support the upper body, a number of structural changes were made to the pelvis. The ilial pelvic bone shifted forward and broadened, while the ischial pelvic bone shrank, narrowing the pelvic canal. These changes were occurring at the same time as humans were developing larger craniums. Male versus female Examination of the pelvis is the most useful method for identifying biological sex through the skeleton. Distinguishing features between the human male and female pelvis stem from the selective pressures of childbearing and birth. Females must be able to carry out the process of childbirth but also be able to move bipedally. The human female pelvis has evolved to be as wide as possible while still being able to allow bipedal locomotion. The compromise between these two necessary functions of the female pelvis can be especially seen through the comparative skeletal anatomy between males and females. One of the last key differences can be seen in the auricular surface of the pelvic bones. The auricular surface where the sacroiliac joint articulates seen in females generally has a rougher texture compared to the surfaces seen in males. In modern humans, parturition (childbirth) differs greatly from the rest of the primates because of both the pelvic shape of the mother and the neonatal shape of the infant. Further adaptations evolved to cope with bipedalism and larger craniums were also important such as neonatal rotation of the infant, shorter gestation length, assistance with birth, and a malleable neonatal head. Neonatal rotation Neonatal rotation was a solution for humans evolving larger brain sizes. Comparative zoological analysis has shown that the size of the human brain is anomalous, as humans have brains that are significantly larger than other animals of similar proportions. Even among the great apes, humans are distinctive in this regard, having brains three to four times larger than those of chimpanzees, humans' nearest relatives. Although the close correspondence between the neonatal cranium and the maternal pelvis in monkeys is also characteristic of humans, the orientation of the pelvic diameters differs. On average, a human fetus is nearly twice as large in relation to its mother's weight as would be expected for another similarly sized primate. During delivery, neonatal rotation occurs when the body gets rotated to align head and shoulders transversely when entering the small pelvis, otherwise known as internal rotation. The fetus then rotates longitudinally to exit the birth canal, which is known as external rotation. In humans, the long axes of the inlet and the outlet of the obstetric canal lie perpendicular to each other. These movements include engagement, descent, flexion, internal rotation, extension, external rotation, and expulsion. • Engagement is the first movement of labor, where the first part of the head enters the pelvic inlet. • Descent refers to the deeper movement of the head through the pelvic inlet with the widest diameter of the infant's head. • Flexion occurs during descent, where tissues of the pelvis create resistance as the head moves down the pelvic cavity and brings the infant's chin to the chest. This allows for the smallest part of the head to begin to push through the pelvis and actively promote delivery of the baby. • Internal rotation occurs when the head continues to descend and comes in contact with the pelvic floor, which has resistant muscles. These muscles allow for the infant to rotate their head to allow their head and shoulders to move through the pelvis. Due to the broad shape of the sacrum, the head of the fetus must be rotated from occiput transverse to occiput anterior position, which means the infant must rotate from the sideways position so the anterior head faces the buttock of the mother. Comparative data from across mammals and primates suggest that there is a metabolic constraint on how large and energetically expensive a fetus can grow before it must leave the mother's body. Scientists have believed that the shorter gestation period can be attributed to the narrower pelvis, as the baby must be born before its head reaches a volume that cannot be accommodated by the obstetric canal. Social assistance Human infants are also almost always born with assistance from other humans because of the way that the pelvis is shaped. Since the pelvis and opening of the birth canal face backwards, humans have difficulty giving birth themselves because they cannot guide the baby out of the canal. Non-human primates seek seclusion when giving birth because they do not need any help due to the pelvis and opening being more forward. Human infants depend on their parents much more and for much longer than other primates. So in humans, the cost of slow development of their infants is that humans reproduce relatively slowly. This phenomenon is also known as cooperative breeding. Malleable cranium Humans are born with a very malleable fetal head which is not fully developed when the infant exits the womb. This soft spot on the crown of the infant allows for the head to be compressed in order to better fit through the birth canal without obstructing it. This allows for the head to develop more after birth and for the cranium to continue growing without affecting the birthing process. ==Challenges to the obstetrical dilemma hypothesis==

The obstetrical dilemma hypothesis has had several challenges to it, as more data is collected and analyzed. Several different fields of study have taken an interest in understanding more about the human birth process and that of human ancestor species. Early brain growth rates Some studies have shown that higher brain growth rates happen earlier on in ontogeny than previously thought, which challenges the idea that the explanation of the obstetrical dilemma is that humans are born with underdeveloped brains. This is because if brain growth rates were largest in early development, that is when the brain size would increase the most. Premature birth would not allow for a much larger head size if most of the growth had already happened. Also, it has been suggested that maternal pelvic dimensions are sensitive to some ecological factors. Maternal heat stress There has been a lot of evidence linking body mass to brain mass, leading to the determination of maternal metabolism as a key factor in the growth of the fetus. Maternal constraints could be largely due to thermal stress or energy availability. A larger brain mass in the neonate corresponds to more energy needed to sustain it. It takes much more energy for the mother if the brain fully develops in the womb. If maternal energy is the limiting factor, then an infant can only grow as much as the mother can sustain. Also, because fetal size is positively correlated to maternal energy use, thermal stress is an issue because the larger the fetus, the more the mother can suffer heat stress. Another study disproves the thought that narrower hips are optimized for locomotion because it was found that a Late Stone Age population in Southern Africa that survived largely on terrestrial mobility had women who had uncharacteristically small body size with large pelvic canals. Additionally, this hypothesis demonstrates that, contrary to the obstetrical dilemma, an increased pelvic size would not be deleterious to bipedalism. Studying the running mechanics of males and females, it was shown that an increased pelvic size related to neither an increased metabolic nor structural demand on a woman. Obstetrical dilemma revisited The obstetrical dilemma hypothesis has also been challenged conceptually based on new studies. The authors argue that the obstetrical dilemma hypothesis assumes that human, and therefore hominid, childbirth has been a painful and dangerous experience through the species' evolution. This assumption may be fundamentally false as many early analyses focused on maternal death data from primarily females of European-descent in Western Europe and the United States during the 19th and 20th century, a limited population. A recent study aimed to evaluate the original ideas under the 'obstetrical dilemma' and provide a detailed, more complex explanation for the tight fetopelvic fit observed in humans. They propose that the original obstetrical dilemma hypothesis remains valuable as a foundation to explain the complex combination of evolutionary, ecological, and biocultural pressures that constrain maternal pelvic form and fetal size. == See also ==