Biomechanical trade-offs in the pelvic floor constrain the evolution of the human birth canal

Abstract

Compared with most other primates, humans are characterized by a tight fit between the maternal birth canal and the fetal head, leading to a relatively high risk of neonatal and maternal mortality and morbidities. Obstetric selection is thought to favor a spacious birth canal, whereas the source for opposing selection is frequently assumed to relate to bipedal locomotion. Another, yet underinvestigated, hypothesis is that a more expansive birth canal suspends the soft tissue of the pelvic floor across a larger area, which is disadvantageous for continence and support of the weight of the inner organs and fetus. To test this “pelvic floor hypothesis,” we generated a finite element model of the human female pelvic floor and varied its radial size and thickness while keeping all else constant. This allowed us to study the effect of pelvic geometry on pelvic floor deflection (i.e., the amount of bending from the original position) and tissue stresses and stretches. Deflection grew disproportionately fast with increasing radial size, and stresses and stretches also increased. By contrast, an increase in thickness increased pelvic floor stiffness (i.e., the resistance to deformation), which reduced deflection but was unable to fully compensate for the effect of increasing radial size. Moreover, larger thicknesses increase the intra-abdominal pressure necessary for childbirth. Our results support the pelvic floor hypothesis and evince functional trade-offs affecting not only the size of the birth canal but also the thickness and stiffness of the pelvic floor.

The obstetrical dilemma hypothesis: there's life in the old dog yet

“…material tissue properties) constant, and found that larger pelvic floors, as defined by their radial dimensions, show disproportionately large downward deformations compared to smaller pelvic floors. These findings corroborate the hypothesis that a larger bony pelvic canal results in a relatively higher degree of pelvic floor deformation under the same intra‐abdominal pressure, which undermines the support function of the pelvic floor and may increase the risk of pelvic floor disorders, such as prolapse (Stansfield et al ., 2021 ).…”

The variation in shape and thickness of the pelvic floor musculature in males and females: a geometric-morphometric analysis

“…Similar associations between a large lower pelvic canal and increased risk of PFDs were reported by other studies [ 7 ]. Also, previous biomechanical modelling showed that a large pelvic floor provides weaker support and descends deeper on application of intra-abdominal pressure than does a smaller one [ 9 ]. In agreement with these clinical and biomechanical results, we found that a relatively larger lower pelvic canal in females was associated with a deeper pelvic floor and a wider puborectalis sling (Fig.…”

“…There exists some evidence in the literature that women with a broader pelvic canal are more prone to developing incontinence and prolapse [ 7 ], but other studies failed to find such associations [ 8 ]. In a finite element analysis, Stansfield et al [ 9 ] modelled the female pelvic floor muscles and demonstrated that a larger pelvic canal results in higher values of displacement, stress and strain in the pelvic floor muscles, which could contribute to a higher risk of PFD. In the same study, they showed that this effect is partially ameliorated by increased pelvic floor muscle thickness.…”

Human shoulder development is adapted to obstetrical constraints

“…Various hypotheses have been proposed to explain this pattern. It has been suggested that more expansive pelvic dimensions could reduce the energetic efficiency of bipedal locomotion ( 1 – 5 ), could also be disadvantageous for bearing the weight of the visceral organs and the fetus during pregnancy ( 6 – 10 ), and may increase the risk of knee and ankle injuries due to increased stress along the mediolateral direction ( 11 ). The resulting obstetrical dilemma ( 12 ) has been hypothesized to persist in modern humans because of opposing and incompatible selective trends favoring large neonatal heads and narrow maternal birth canals ( 7 , 10 , 13 , 14 ).…”

“…It has been suggested that more expansive pelvic dimensions could reduce the energetic efficiency of bipedal locomotion ( 1 – 5 ), could also be disadvantageous for bearing the weight of the visceral organs and the fetus during pregnancy ( 6 – 10 ), and may increase the risk of knee and ankle injuries due to increased stress along the mediolateral direction ( 11 ). The resulting obstetrical dilemma ( 12 ) has been hypothesized to persist in modern humans because of opposing and incompatible selective trends favoring large neonatal heads and narrow maternal birth canals ( 7 , 10 , 13 , 14 ). Recently, the obstetrical dilemma hypothesis was challenged by various studies, indicating that locomotor cost is not increased by wider pelvic dimensions and proposing that fetal brain growth is constrained by the limits of maternal metabolism rather than by obstetrical constraints ( 15 , 16 ).…”