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.2018 Oct 24;285(1889):20181807.
doi: 10.1098/rspb.2018.1807.

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

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

Lia Betti et al. Proc Biol Sci..

Abstract

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 encephalized 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.

Keywords: birth canal; climate; human; neutral variation; obstetrical constraints; pelvis.

© 2018 The Author(s).

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
Pelvic canal measurements (a) and population samples (b) used in this study. A = anteroposterior (AP) diameter of the inlet, from the sacral promontory to the dorsomedial superior pubis; B = AP diameter of the midplane, from the junction of the fourth and fifth sacral vertebrae to the dorsomedial inferior pubis; C = AP diameter of the outlet, from the apex of the fifth sacral vertebra to the dorsomedial inferior pubis; D = mediolateral (ML) diameter of the inlet, as maximum distance between the linea terminalis; E = ML diameter of the midplane, as distance between the ischial spines (often approximated due to damage to the spines); F = ML diameter of the outlet between the inner margins of the transverse ridge of the ischial tuberosities. (Online version in colour.)
Figure 2.
Figure 2.
Variation in the inlet (a), midplane (b), and outlet (c) indices within four major geographical regions. The boxes represent the interquartile distance, the whiskers extend to the most extreme data point which is no more than 1.5 times the interquartile range, and outliers are highlighted as open circles. The ovals on the left represent the extremes of shape observed in this study for each of the canal planes. The endpoints of the colour bars link pairs of regions with significantly different canal shapes (see results ofpost hoc Tukey tests in electronic supplementary material, table S4).
Figure 3.
Figure 3.
Three-dimensional plot of population averages for the inlet, midplane, and outlet indices (a) and coefficient of variation (CV) for various postcranial indices and measurements (b). Populations from four geographical regions are highlighted in different colours in (a) (Africa, green; Asia, yellow; Europe and North Africa, red; Americas, blue). (b) The CV for the birth canal (in blue; this study) and other postcranial indices and measurements (in white; Goldman dataset). Indices are highlighted by an asterisk; pelvis/femur refers to pelvic breadth divided by femoral length. The 95% confidence interval obtained after a jackknife procedure is shown in red.
Figure 4.
Figure 4.
Graphical representation of the key results. (a) Plot of within-population phenotypic distance and geographical distance from central sub-Saharan Africa; (b) plot of between-population phenotypic distance and genetic distance; (c) plot of the population average inlet index against minimum temperature; (d) plot of the residuals of the regression of canal size on body mass, against body mass. (Online version in colour.)
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