Most women have a birth canal in which the upper part, the pelvic inlet, is round or transverse oval. This shape is probably optimal for birth. However, it is unclear why the lower part of the birth canal is longitudinal oval. Because of this twisted shape, the baby must make a screwing motion to pass through the birth canal during birth. This increases the risk of birth complications. The research team led by biomechanist Katya Stansfield has now been able to show that the longitudinal oval shape of the lower birth canal is probably of significant advantage for the stability of the pelvic floor.
Compared to humans, birth in great apes is a relatively simple process: Their birth canal has a longitudinal oval shape in both the pelvic inlet and the outlet. This shape does not require the baby to turn. "For birth, a uniformly shaped birth canal would be beneficial in our species as well," Stansfield said. Instead, the twisted human birth canal requires a more complex birth mechanism: the baby must turn to align the longest diameter of its head with the longest extent of each of the different planes of the birth canal. Misalignment can lead to birth arrest and pose health risks to both mother and baby.
Publication in BMC Biology:
Stansfield, E., Fischer, B., Grunstra, N.D.S. Vila Pouca, M., Mitteroecker, P. The evolution of pelvic canal shape and rotational birth in humans. (2021)
