The evolution of pelvic canal shape and rotational birth in humans

Abstract

Background The human foetus typically 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 of the birth canal has a pronounced anteroposteriorly oval shape. Results Here, we show that the shape of the lower birth canal affects the ability of the pelvic floor to resist the 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. Conclusions 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 the mediolateral elongation has evolved because of the limits on the anteroposterior diameter imposed by upright posture. We show that an anteroposteriorly deeper inlet would require greater pelvic tilt and lumbar lordosis, which compromises spine health and the stability of upright posture. These different requirements of the pelvic inlet and outlet likely have led to the complex shape of the pelvic canal and to the evolution of rotational birth characteristic of humans.

Variation in human 3D trunk shape and its functional implications in hominin evolution

“…For example, the ribcages of Homo erectus and Neandertals were not only wider at the level of the central and lower thorax, but also antero-posteriorly deeper than most modern human populations [1][2][3][4] . Also the pelvis shows a systemic evolutionary trend towards reduction of its bi-iliac width, when comparing modern humans with H. erectus and members of the Neandertal lineage [5][6][7][8] . Evolutionary changes in the spine of the genus Homo show changes in overall height, it's position within the ribcage and possibly spine curvatures.…”

“…Besides a positive effect of overall trunk muscularity 26 , 30 on running performance, it has been shown that several other specific trunk morphological aspects relate to running performance, including the width of the pelvis 8 , 31 , the trunk flexion angle 32 , lumbar lordosis 33 , 34 and associated hip flexion 31 , and thorax breathing mechanics 35 , 36 .…”

Distocia de hombros: propuestas de resolución de acuerdo con las diferentes posiciones de parto según la movilidad de la pelvis

“…Por otra parte, estos planos presentan diferentes orientaciones y diámetros que están delimitados por los rebordes óseos y las estructuras músculo-aponeuróticas. Esto obliga al feto a realizar no solamente un trayecto curvo en su descenso, sino además una rotación similar a un movimiento de tornillo de ¼ a ¾ de vuelta (8,9) . De forma esquemática, el canal del parto puede analizarse a partir de una entrada o espacio superior, un espacio medio y una salida o espacio inferior.…”

“…Los hombros deben seguir el movimiento de la cabeza realizando su primera rotación: desde la posición sagital o anteroposterior a la posición transversa u oblicua. Si el dorso es izquierdo, el hombro anterior o derecho se relaciona con la línea pectínea derecha y el hombro posterior o izquierdo se relaciona con la línea innominada izquierda (9) . Este último es el primero que debe penetrar en la pelvis, pues es el más distante de la salida y el más sometido a la tracción.…”

Hip width and metabolic energy expenditure of abductor muscles

“…That is, the assumption at the heart of the OD—that pelvic width is constrained by locomotor energy expenditure—is unsupported by convincing empirical evidence in modern humans. Sufficient questions remain to be answered, however, before the metabolic energy clause of the OD can be completely rejected [ 11 ] and other potential causes (e.g., pelvic floor integrity [ 31 , 32 ], femoral neck fracture risk [ 33 ] emphasized.…”