In most ladies, the upper part, or inlet, of the birth canal has a round or transversely (left-to-right) oval shape, which is considered ideal for parturition, but it is unidentified why the lower part of the birth canal has a noticable longitudinally (front-to-back) oval shape. This twisted shape normally requires the Baby to rotate when passing through the narrow birth canal, which further increases the danger of birth complications.
In comparison with humans, apes have a fairly simple birth pattern that does not require rotation of the infant thanks to the longitudinally oval shape of the birth canal both at its inlet and the outlet. Instead, the twisted human shape requires a complex, rotational birth mechanism: The child needs to turn to line up the longest dimension of its head with the best dimension of each aircraft of the birth canal.
A research team of evolutionary biologists and engineers from the University of Vienna, the Konrad Lorenz Institute for Evolution and Cognition Research in Klosterneuburg and the University of Porto hypothesized that the assistance function of the pelvic flooring muscles, which are suspended across the lower hips and also play a crucial role in sexual function and continence, may have influenced the advancement of the shape of the birth canal. The group carried out substantial biomechanical modeling of the pelvic floor and found that the highest pressure, deformation, and stress take place in pelvic floorings with a circular or transverse-oval shape, whereas a longitudinally oval elongation increases pelvic flooring stability. “Our results show that the longitudinally oval lower birth canal is useful in regards to stability,” says Katya Stansfield. “However, this result prompted us to ask why the pelvic inlet in people is not likewise lengthened longitudinally,” elaborates Barbara Fischer, an evolutionary biologist.
“We argue that the transverse elongation of the pelvic inlet has progressed because of the limits on the front-to-back size in human beings enforced by balancing upright posture, rather than by the performance of the bipedal mobility,” states Philipp Mitteroecker, who was likewise included in this research study. A longitudinally much deeper inlet would require greater pelvic tilt and lumbar lordosis, which would jeopardize spinal column health and the stability of upright posture.
Referral: “The evolution of pelvic canal shape and rotational birth in human beings” by Ekaterina Stansfield, Barbara Fischer, Nicole D. S. Grunstra, Maria Villa Pouca and Philipp Mitteroecker, 11 October 2021, BMC Biology.DOI: 10.1186/ s12915-021-01150-w.
In contrast with humans, apes have a fairly easy birth pattern that does not require rotation of the baby thanks to the longitudinally oval shape of the birth canal both at its inlet and the outlet. “For providing birth, it would be much simpler to have a consistently shaped birth canal likewise in our species,” says Katya Stansfield, a specialist in biomechanics. Instead, the twisted human shape requires a complex, rotational birth mechanism: The baby requires to turn to line up the longest dimension of its head with the widest measurement of each plane of the birth canal.
Extraordinary shape makes births more hard, but warranties stability.
The reasonably narrow human birth canal presumably progressed as a “compromise” between its capabilities for parturition, support of the inner organs, and upright walking. However not just the size of the birth canal, likewise 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 providing new insights into why the human birth canal evolved to have this complex shape. They suggest that the longitudinally oval shape of the lower birth canal is useful for the stability of the pelvic flooring muscles.