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 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.