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

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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 under-investigated, 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.

Original languageEnglish
Article numbere2022159118
Number of pages8
JournalProceedings of the National Academy of Sciences of the United States of America (PNAS)
Issue number16
Publication statusPublished - 20 Apr 2021

Austrian Fields of Science 2012

  • 106018 Human biology
  • 106056 Biological anthropology
  • 106045 Theoretical biology


  • pelvic floor
  • human birth canal
  • biomechanics
  • evolutionary trade-off
  • finite element modeling
  • Evolutionary trade-off
  • Biomechanics
  • Pelvic floor
  • Finite element modeling
  • Human birth canal

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