Why do people have a twisted birth canal?


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The relatively narrow human birth canal presumably evolved as a “compromise” between its ability to give birth, support for the internal organs, and upright gait. But not only the size of the birth canal, but also its complex, “twisted” shape is an evolutionary puzzle. Katya Stansfield from the University of Vienna and her co-authors have published a study in BMC Biology presents new insights into why the human birth canal evolved to have this complex form. They suggest that the longitudinal oval shape of the lower birth canal is beneficial for the stability of the pelvic floor muscles.

In most women, the upper part, or inlet, of the birth canal has a round or transverse (left-to-right) oval shape, which is considered ideal for childbirth, but it is unknown why the lower part of the birth canal has a pronounced longitudinal (front-to-back) oval shape. This twisted shape typically requires the baby to rotate as it passes through the narrow birth canal, further increasing the risk of birth complications.

In comparison with humans, monkeys have a relatively light birth pattern that does not require rotation of the baby thanks to the longitudinal oval shape of the birth canal both at its inlet and outlet. “To give birth, it will be much easier to have a uniformly shaped birth canal in our species as well,” says Katya Stansfield, a specialist in biomechanics. Instead, the twisted human shape requires a complex, rotating birth mechanism: The baby must rotate to align the longest dimension of its head with the widest dimension of each plane of the birth canal. Misadjustment can lead to blocked birth and result in health risks for both mother and baby.

A research team of evolutionary biologists and engineers from the University of Vienna, the Konrad Lorenz Institute for Evolution and Cognitive Research in Klosterneuburg and the University of Porto assumed that the support function of the pelvic floor muscles, which is suspended across the lower pelvis and also plays an important role in sexual function and continence, may have influenced the development of the shape of the birth canal. The team performed extensive biomechanical modeling of the pelvic floor and found that the highest deformation, stress, and strain occur in pelvic bottoms with a circular or transverse oval shape, whereas a longitudinal oval extension increases pelvic floor stability. “Our results show that the longitudinal oval lower birth canal is beneficial in terms of stability,” says Katya Stansfield. “But this result made us ask why the pelvic inlet in humans is not also lengthened in the longitudinal direction,” explains Barbara Fischer, an evolutionary biologist.

Traditionally, it has been assumed that the transverse dimension of the human pelvis is limited by the efficiency of upright movement. “We argue that the transverse extension of the pelvic inlet has evolved due to the limits of the front-to-back diameter in humans, which are imposed by balancing upright position, rather than by the efficiency of the bipedal movement,” says Philipp Mitteroecker, who was also involved in this study. A deeper longitudinal inlet would require greater pelvic tilt and lumbar lordosis, which would compromise the health of the spine and the stability of the upright position. These different requirements for pelvic inlets and outlets have likely led to the development of a twisted birth canal that requires human babies to rotate during birth.

Birth versus pelvic floor stability

More information:
Ekaterina Stansfield et al., The evolution of pelvic canal shape and rotational birth in humans, BMC Biology (2021). DOI: 10.1186 / s12915-021-01150-w

Provided by the University of Vienna

Citation: Why do humans possess a twisted birth canal? (2021, October 29) retrieved October 30, 2021 from https://phys.org/news/2021-10-humans-birth-canal.html

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