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Obstetrical dilemma
The obstetrical dilemma refers to two conflicting trends in the evolutionary development of the human pelvis where the transition to walking upright, bipedal locomotion, required a decreased size of the bony birth-canal but the trend toward increased intelligence required a larger cranium, which would need a wider obstetrical pelvic area.〔Wittman, A. B.; Wall, L. L. (2007). "The Evolutionary Origins of Obstructed Labor: Bipedalism, Encephalization, and the Human Obstetric Dilemma". Obstetrical & Gynecological Survey 62 (11): 739.〕 The obstetrical dilemma has been studied widely because it has increased the difficulty of childbirth for modern day women. Because of this, complications in birth are more frequent as well as the need of assistance in childbirth. This change seems to have evolved over time because this dilemma does not seem to pose a problem for our most recent relatives, non-human primates, who still manage to give birth with little difficulty.
==Evolution==
Human ancestors seemed to originally give birth in a similar way that non-human primates do. Most primates have neonatal heads that are close in size to the mother’s birth canal. Because of this females do not seem to need assistance in birthing and seek seclusion.〔Isler, Karin, van Schaik, Caret. 2012. Allomaternal care, life history, and brain size evolution in mammals. Journal of Human Evolution 63: 52-63〕 There is also no need for neonatal rotation because the canals are wide enough to accommodate the infant.〔Rosenberg, Karen, Trevathan, Wenda. 2003. Birth, obstetrics and human evolution. An International Journal of Obstetrics and Gynaecology 109 (11): 1199-1206〕 Modern humans birthing methods greatly differ from this because of both pelvic shape of the mother and neonatal shape of the infant.
The obstetrical dilemma began when human ancestors started to evolve into a bipedal creature. Because humans are the only obligately bipedal primates, meaning their body shape requires them to only use two legs, major alterations had to be made to the shape of the female pelvis.〔 A number of structures in the body 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. Therefore, in order to successfully undergo childbirth, the infant must be born earlier and earlier, thereby making the child increasingly developmentally premature.〔Dunsworth, H. M.; Warrener, A. G.; Deacon, T.; Ellison, P. T.; Pontzer, H. (2012). "Metabolic hypothesis for human altriciality". Proceedings of the National Academy of Sciences 109 (38): 15212〕 The concept of the infant being born underdeveloped is called altriciality. Other ways of evolving to cope with bipedalism and larger craniums were also important such as neonatal rotation of the shoulders to allow the infant to fit through the canal, shorter gestation length which allows the infant to be born smaller, assistance with birth, and a malleable neonatal head which is softer and leaves the birth canal more easily.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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