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.[1]
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).[2] 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.[3][4]
This idea was widely accepted when first published in 1960, but has since been criticized by other scientists.[5]
^Say, Lale; Chou, Doris; Gemmill, Alison; Tunçalp, Özge; Moller, Ann-Beth; Daniels, Jane; Gülmezoglu, A Metin; Temmerman, Marleen; Alkema, Leontine (June 2014). "Global causes of maternal death: a WHO systematic analysis". The Lancet Global Health. 2 (6): e323–e333. doi:10.1016/S2214-109X(14)70227-X. hdl:1854/LU-5796925. PMID 25103301. S2CID 8706769.
^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): 739–748. doi:10.1097/01.ogx.0000286584.04310.5c. PMID 17925047. S2CID 9543264.
^Pavličev, Mihaela; Romero, Roberto; Mitteroecker, Philipp (January 2020). "Evolution of the human pelvis and obstructed labor: new explanations of an old obstetrical dilemma". American Journal of Obstetrics and Gynecology. 222 (1): 3–16. doi:10.1016/j.ajog.2019.06.043. PMC 9069416. PMID 31251927.
^Stansfield, Ekaterina; Kumar, Krishna; Mitteroecker, Philipp; Grunstra, Nicole D. S. (2021-04-20). "Biomechanical trade-offs in the pelvic floor constrain the evolution of the human birth canal". Proceedings of the National Academy of Sciences. 118 (16): e2022159118. Bibcode:2021PNAS..11822159S. doi:10.1073/pnas.2022159118. ISSN 0027-8424. PMC 8072325. PMID 33853947.
^Glausiusz, Josie (2017-11-29). "Of Evolution, Culture, and the Obstetrical Dilemma". Undark Magazine. Retrieved 2019-11-24.
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