The human birth canal is just large enough for the newborn, making childbirth a difficult and often dangerous process. In a recent study published in the Royal Society, Lia Betti (University of Roehampton) and Andrea Manica (University of Cambridge) show that there is substantial variation in the shape of the female pelvis across human populations, and that most of the differences among geographic regions can be best explained by migration of humans across the globe (and genetic variation accumulated along the way). In other words, the large geographic variation we see is largely random and non functional, although natural selection in the form of climatic adaptation has played a minor role.
These results are important for our understanding of human evolution, especially in challenging the leading theory (the obstetrical dilemma theory) that explains human difficulties in giving birth as the evolutionary consequence of opposing adaptations to bipedal locomotion (favouring a narrow pelvis) and childbirth of large-headed babies (favouring a spacious pelvis). Based on this theory, we would expect the pelvic canal to be quite constrained towards a shape that provides a compromise between the two pressures, while we instead see considerable variation in shape.
The results are also important for obstetric training and practice in modern multiethnic societies, with London a prime example. If women of different ancestries tend to have differently shaped pelves, it is possible (and indeed there is some evidence of this) that they go through labour differently. Modern obstetric training and practice in Europe is based on classic 20th century studies of (mostly) white women, and a revision of textbooks and guidelines might be needed to include the wider spectrum of pelvic shape diversity shown by this paper.
To date the study has been covered in the following media outlets:
- Science, 23 October 2018, “Birth canals are different all over the world, countering a long-held evolutionary theory” (by Erica Tennenhouse).
- Agence France Presse, 24 October 2018, “Women’s birth canals in Kenya, Korea, Kansas not the same: study” / “Le canal que suit un bébé pendant l’accouchement diffère selon l’origine de la mère (étude)” (by Marlowe Hood).
- The Guardian, 24 October 2018, “Focus on western women ‘skewed our ideas of what birth should look like’” (by Nicola Davis).
- The Scientist, 24 October 2018, “Human birth canal varies more widely than previously thought” (by Anna Azvolinsky).
- The New York Times, 27 October 2018, “Why textbooks may need to update what they say about birth canals” (by Steph Yin).
- BBC World Service, World Update (interviewed by Julian Marshall), 24 October 2018.
- BBC World Service, Newshour (interviewed by Rasia Iqbal), 24 October 2018.
- Radio France Internationale, “New birth canal research could improve quality & safety for non-European mothers” (interviewed by Laura Angela Bagnetto), 24 October 2018.