Geographic variation in the human birth canal and its implications for the ‘obstetrical dilemma’ theory

Childbirth, from H. Deventer “Operationes Chirurgicae” (c) Wellcome Trust

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:

Radio interviews have also been conducted in the following outlets:

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