Obstetrical dilemma

Hypothesis about human childbirth
The obstetrical dilemma is a hypothesis to explain why humans often require assistance from other humans during childbirth to avoid complications, whereas most non human primates give birth unassisted with relatively little difficulty. This occurs due to the tight fit of the fetal head to the maternal birth canal, which is additionally convoluted, meaning the head and therefore body of the infant must rotate during childbirth in order to fit, unlike in other, non upright walking mammals. Consequently, there is an unusually high incidence of cephalopelvic disproportion and obstructed labor in humans. The obstetrical dilemma claims that this difference is due to the biological trade off imposed by two opposing evolutionary pressures in the development of the human pelvis: smaller birth canals in the mothers, and larger brains, and therefore skulls in the babies. Proponents believe bipedal locomotion (the ability to walk upright) decreased the size of the bony parts of the birth canal. They also believe that as hominids' and humans' skull and brain sizes increased over the millennia, that women needed wider hips to give birth, that these wider hips made women inherently less able to walk or run than men, and that babies had to be born earlier to fit through the birth canal, resulting in the so called fourth trimester period for newborns (being born when the baby seems less developed than in other animals). Recent evidence has suggested bipedal locomotion is only a part of the strong evolutionary pressure constraining the expansion of the maternal birth canal. In addition to bipedal locomotion, the reduced strength of the pelvic floor due to a wider maternal pelvis also leads to fitness detriments in the mother pressuring the birth canal to remain relatively narrow. This idea was widely accepted when first published in 1960, but has since been criticized by other scientists.

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.

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 to the shape of the female pelvis. Human pelvises have no 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 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.

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 pelvic shape of the mother and 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 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 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 pelvic. 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.

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 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.

Humans are born with a very malleable fetal head which is not fully developed when the infant exits the womb. 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.

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 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.

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 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.