Human variation in the shape of the birth canal is significant and geographically structured

Lia Betti, Andrea Manica

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The human birth canal shows a tight fit with the size of the neonate, which can lead to obstetric complications. This is not the case in other apes, and has been explained as the outcome of conflicting evolutionary pressures for bipedal locomotion and parturition of a highly-encephalised fetus. Despite the suggested evolutionary constraints on the female pelvis, we show that women are, in fact, extremely variable in the shape of the bony birth canal, with human populations having differently-shaped pelvic canals. Neutral evolution through genetic drift and differential migration are largely responsible for the observed pattern of morphological diversity, which correlates well with neutral genetic diversity. Climatic adaptation might have played a role, albeit a minor one, with populations from colder regions showing a more transversally oval shape of the canal inlet. The significant extent of canal shape variation among women from different regions of the world has important implications for modern obstetric practice in multi-ethnic societies, as modern medical understanding has been largely developed on studies of European women.

© 2018, The Author(s) Published by the Royal Society. This is an author produced version of a paper published in PROCEEDINGS OF THE ROYAL SOCIETY B BIOLOGICAL SCIENCES uploaded in accordance with the publisher’s self- archiving policy. The final published version (version of record) is available online at the link below. Some minor differences between this version and the final published version may remain. We suggest you refer to the final published version should you wish to cite from it.
Original languageEnglish
Article number20181807
JournalProceedings of The Royal Society B Biological Sciences
Issue number1889
Publication statusPublished - 24 Oct 2018


  • pelvis
  • climate
  • neutral variation
  • obstetrical constraints
  • birth canal
  • human evolution

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