Biomechanics of the pelvic floor as a possible evolutionary constraint in humans


Katya Stansfield

Unit for Theoretical Biology, Department of Evolutionary Biology
University of Vienna

The difficulty of the human childbirth is well documented with the obstructed labour occurring in up to 6% of all births and up to 30% of women experiencing pelvic floor disorders due to the vaginal delivery. Mitteroecker et al. (2017) have shown that only a small selection for the larger infant and/or the smaller pelvis is enough to maintain the poor birth outcomes in a small part of the population. The mechanism of this selection is, however, hotly debated. One of the hypotheses suggests that the demands put on the pelvic floor tissue by its supporting role may create such a factor. Unlike in apes and monkeys, human pelvic floor must withstand a higher hydrostatic pressure continuously and provide for the continence, while maintaining voluntary sphincter control.

In the present study, we investigate how changing the healthy pelvic floor morphology affects its performance. We demonstrate that the decrease in the thickness of the healthy tissue results in a dramatic increase of the pelvic floor deflection below the 3mm level. At the same time, the increase in the PF size brings a disproportional increase in its displacement, which may undermine its support function. Furthermore, we show that thickness has buffering effect on displacement, which implies that an evolutionary increase in birth canal size, and thus in the surface area of the pelvic floor, need not compromise pelvic floor support. However, to achieve that, the pelvic floor thickness must increase faster than the surface area. But while such an increase in thickness would be beneficial for continence and support of the gravid uterus during pregnancy, it might be disadvantageous for childbirth.