The evolution of pelvic canal shape and rotational birth in humans
Abstract The human foetus needs to rotate when passing through the tight birth canal because of the complex shape of the pelvis. In most women the upper part, or inlet, of the birth canal has a round or mediolaterally oval shape, which is considered ideal for parturition, but it is unknown why the lower part, or outlet, of the birth canal has a pronounced anteroposteriorly oval shape. Here we show that the shape of the lower birth canal affects the ability of the pelvic floor to resist pressure exerted by the abdominal organs and the foetus. Based on a series of finite element analyses, we found that the highest deformation, stress and strain occur in pelvic floors with a circular or mediolaterally oval shape, whereas an anteroposterior elongation increases pelvic floor stability. This suggests that the anteroposterior oval outlet shape is an evolutionary adaptation for pelvic floor support. For the pelvic inlet, by contrast, it has long been assumed that the mediolateral dimension is constrained by the efficiency of upright locomotion. But we argue that upright stance limits the anteroposterior dimension of the inlet. A deeper inlet requires greater pelvic tilt and lumbar lordosis, which compromises spine health and the stability of upright posture. These different requirements on the pelvic inlet and outlet have led to the complex shape of the human pelvic canal and to the evolution of rotational birth.