The broad baby head has to squeeze through an astonishing narrowness – why hasn’t the human birth canal just become a little wider in the course of evolution? A study now shows that an anatomical compromise, which has to do with our upright posture, contributed to the human birth problems. One problem was evidently the function of the pelvic floor: In order to provide adequate support and at the same time enable childbirth, a “It couldn’t be better” has developed in the characteristics of this tissue, according to model simulations.
In dogs, cats and the like, as well as in great apes, births are usually unproblematic. In comparison, it is a drama in humans: births are not only painful, but also involve very high risks for mother and child. The reason for this is a birth canal that is narrow compared to animals in relation to the large heads of the babies. The fact that the human birth canal has not simply become wider in the course of evolution is already attributed to compromises associated with our upright gait. The relatively narrow birth canal is therefore the result of an evolutionary “tug-of-war” between different opposing selection forces. The so-called pelvic floor hypothesis states that an even wider birth canal would critically weaken the support function of the woman’s pelvic floor muscles, which would lead to increased “sagging”.
The pelvic floor in sight
The pelvic floor is an elastic structure that extends from the tailbone to the pubic bone. The muscle tissue there protects the organs, including the uterus, bladder and intestines, from sagging. This support function is also particularly important to give the child support during pregnancy. The increased incidence of pelvic floor disorders such as various forms of incontinence in women was already considered an indication of a connection between a wide pelvis and a weakening of the pelvic floor. But so far the pelvic floor hypothesis has not been specifically tested in connection with the evolution of the female anatomy.
To close this gap, the scientists led by Nicole Grunstra from the University of Vienna simulated human pelvic floors of various sizes on the computer. “This approach enabled us to look at the effect of pool size on sagging in isolation without having to consider data from living people,” says Grunstra. The team used a simulation technique normally used in construction to test the extent to which structures can withstand high pressure and stress. In this case, this so-called finite element analysis allowed the team to model the pelvic floor, change its parameters and see how this works. “You can compare processes in connection with the pelvic floor with loads like a landslide,” says co-author Krishna Kumar from the University of Texas at Austin.
Further – and also thicker is not possible
The team was initially able to confirm: The deflection – the sagging – of the pelvic floor increased disproportionately when the pelvic floor area increased as a result of a wider pelvis. For the first time, the researchers were able to use simulation data to support the fact that the female anatomy has to create a balance between birth and pelvic floor support. In addition, they were able to further develop the pelvic floor hypothesis. Because they also investigated the question of why a thicker pelvic floor was not developed to compensate for the tendency to sag.
The corresponding simulations showed: An increasingly stronger pelvic floor actually leads to a reduced deflection due to its greater rigidity. But not in a favorable way: a larger and thicker pelvic floor still showed a relatively strong tendency to sag compared to a small and thin one. “This means that complete compensation for the deflection of the pelvic floor can only be achieved if the pelvic floor becomes disproportionately much thicker,” says Grunstra. As the scientists explain, a corresponding increase in the strength of the pelvic floor would in turn make the birthing process much more difficult.
This is because the mother’s abdomen has to produce very high pressure in order to stretch the pelvic floor muscles enough so that the child can be born. If the pelvic floor becomes too thick and therefore too stiff, it could be that women can no longer generate the necessary pressure through uterine contractions and pressing, say the researchers. “A thick pelvic floor may be excellent as a support for the internal organs and the fetus, but ultimately the child has to be born, and a pelvic floor that cannot be stretched sufficiently would make the birth dramatically more difficult, even if the birth canal is sufficiently large would be ”, summarizes Grunstra. The researchers believe that they have now uncovered another, previously unknown, functional factor that has influenced the evolution of the human pelvis.
Source: University of Vienna, University of Texas, specialist article: PNAS, doi: 10.1073 / pnas.2022159118