Any scientific conversation on the process of giving birth in medical, veterinary and wild animal husbandry circles ends up on one topic: any mismatch between the size of the offspring to be born and the width of the route out into the world equals disaster: death of the offspring and death of the mother if there is no intervention.
The evolutionary trade-offs involved between maternal genes and paternal genes in an offspring and survival of a mother giving birth are well known. One might have expected that the degree of variation in the shape of the birth canal would not be great, especially in human mothers which have to accommodate the large head typical of the species. However, a recent paper* shows that this is not the case: there is considerable variation in the shape of the birth canal which means that the turns the baby has to make from its position in the uterus is not so simple as once thought.
By measuring the pelvis of skeletons from many parts of the world, Lia Betti (University of Roehampton) and Andrea Manica (Zoology, Cambridge) found:
Sub-Saharan African populations are overall characterized by a deeper birth canal in the anterior–posterior direction, throughout the three planes (inlet, midplane, and outlet), while Native American populations fall at the other extreme of variation with a more transversally wide canal. Asian and European/North African populations show an intermediate morphology. The differences are particularly obvious for the inlet, which tends to be more markedly oval in Americans and Europeans/North Africans, and for the outlet, which tends to be sagittally oval in sub-Saharan Africans and Asians, while it is generally transversally oval in Americans and Europeans/North Africans. It is worth noting, however, that canal variation is continuous and without abrupt differences between regions, when analysed at the level of single populations.
The authors presented evidence that the variation is neutral, i.e. no selective advantage or disadvantage, and is more marked the greater the difference from the presumed ancestral origins of Homo sapiens. There is the possibility, as the authors suggest, that the constraints on pelvic diversity may have been less evident before the adoption of agriculture led to an increase in fetal growth. In other words, the fit between fetal head and maternal pelvis might not have been quite so tight during the initial spread out of Africa. And yes, my 10½ lb birthweight to a non-diabetic mother might be explained by her fondness of food—in quantity.
The authors also point out the consequences for midwifery and obstetric practice of their work:
The magnitude of canal shape variation in human populations revealed by this study sits in stark contrast with the simplified description of the typical human canal morphology in many anatomy books. The description is often based on the most common shape in European individuals, and does not take into account the wide range of variation showed by our species. The rotation movements required by the fetus to negotiate the twisting passage are also generally reported based on an average European experience. Substantial differences in the shape of the canal in modern populations, especially in the outlet, might translate into differences in fetal movements and presentation. Indeed, X-ray studies of labouring women from the first half of the twentieth century provide some evidence of differences in fetal presentation during labour depending on the shape of the mother’s pelvic inlet. The head of the fetus tends to align to the wider diameter of the inlet at engagement. A different rotation of the fetus from the norm might, therefore, occur in women with a differently shaped canal, and should not necessarily be interpreted as a problem. Given the geographical differences in canal shape among modern populations showed by this study, a wider range of variation in childbirth might be expected in modern multi-ethnic societies, and should be taken into account in obstetric training and practice.
I read the paper with considerable interest since an old research activity suddenly loomed large in my mind. The birth canal of eutherian mammals is not limited a rigid bony cage. Relaxation, particularly of the public symphysis but also of the sacro-iliac joints, brought about at least in part by the hormone, relaxin, results in an enlargement of the birth canal, particularly transversely. While the effects of relaxation in the human pelvis are not usually so spectacular as that, say, in the guinea-pig I do have two questions:
1. Are the differences in orientation of the birth canal found by measuring the structure of the pelvis in skeletal remains maintained during parturition?
2. Is there variation in the extent of pelvic relaxation in late pregnancy, and does any such variation correlate with the initial shape of the pelvis?
In short, an important and fascinating piece of research.
Betti L, Manica A. 2018. Human variation in the shape of the birth canal is significant and geographically structured. Proceedings of the Royal Society B 285 20181807. http://dx.doi.org/10.1098/rspb.2018.1807
*The research described was supported in part by the Wenner-Gren Foundation. I wrote about its formation and the role of Paul Fejos in my article Komodo Dragons in the 1930s: a zoo quest before ‘Zoo Quest’ with links to Adolf Hitler, nazi spy scares, the FBI, a cuckolded husband and John F Kennedy of 6 July 2017.
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