The dilemma of human childbirth and the pelvic floor

Unlike in most animals, the birth process in humans is unusually long and painful. Especially if medical care is not available, it can lead to serious injuries and even to the death of both the mother and the baby. The difficulty mainly arises from a very tight fit between the babys head and shoulders and the mothers pelvis. It remains puzzling why the pelvic birth canal has not evolved to be wider in order to ease childbirth. Given the high rates of Caesarean section, this puzzle has immediate consequences for public health also in industrialized countries. It is assumed that the tight fit of the birth canal results from the involvement of the pelvis in two conflicting functions, childbirth and upright walking, giving rise to the so-called obstetrical dilemma. In the present project, we will test an alternative hypothesis: the female pelvic canal has evolved by trading off obstetric demands against support of the pelvic floor. In upright humans, the pelvic floor has to resist the pressure exerted from above by the inner organs and the developing fetus. Pelvic floor disorders thus are frequent in humans, and the medical literature suggests that they are related to pelvic dimensions. For the first time, we will test this hypothesis by a novel combination of methods from medical imaging, biostatistics, and engineering. Based on three-dimensional medical images, we will derive a detailed geometric model of the pelvic floor system, including muscles, tendons and ligaments. With pressure values ranging from normal values at rest to high short-lasting pressure during coughing or jumping, we will create a range of cutting-edge biomechanical models to predict the function and possible impairment of the pelvic floor. These biomechanical models will then be applied to the full adult range of pelvic form as well as extrapolations thereof. This will allow us to infer selective regimes on pelvic form and to test if and how obstetric demands of the female pelvic canal have been traded off against pelvic floor support. This interdisciplinary project will set new methodological standards in evolutionary and functional anatomy. It is likely to break new grounds in evolutionary medicine and will offer novel perspectives in gynaecology and obstetrics.

Unlike in most animals, the birth process in humans is unusually long and painful with high rates of morbidity and mortality among mothers and babies, especially in absence of medical care. The difficulty mainly comes from a very tight fit between the baby's head and shoulders and the mother's pelvis. It remains puzzling why the pelvic canal has not evolved to be wider to ease childbirth. It is assumed that the tight fit of the birth canal results from the two conflicting functions of the pelvis, parturition and bipedal locomotion, giving rise to the so-called 'obstetrical dilemma'. Here, we have tested an alternative hypothesis: size and shape of the female pelvic canal has evolved by trading off obstetric demands against support of the pelvic floor. In erect humans, the pelvic floor resists the pressure exerted from above by the viscera and the developing foetus. Pelvic floor disorders thus are frequent in humans, and the medical literature suggests that they are related to pelvic dimensions. To test this, we applied a novel combination of statistical shape analysis and biomechanical modelling. We found that pelvic floor deflection, stress and strain grow disproportionately fast with increasing radial size of the lower pelvis, provided the shape and thickness of the pelvic floor membrane do not change. However, if thickness of the pelvic floor increases, it resists the deformation better, but the effect of the increased size is not not fully compensated. Moreover, a thicker pelvic floor requires an increase in the intra-abdominal pressure applied during childbirth. Our results support the pelvic floor hypothesis and confirm functional trade-offs affecting not only the size of the birth canal but also the thickness and stiffness of the pelvic floor. At the same time, the transverse shape of the pelvic floor membrane is also important for its support function. We found that a mediolaterally wider pelvic floor experiences higher deflection, stress and strain than an anteroposteriorly longer one. Given that most women have an anteroposteriorly elongated shape of the lower birth canal, our findings suggest that it may have been selected for pelvic floor stability. Our further results show that the shape of the pelvic floor depends on different life parameters in men and in women. Weight has higher impact in men, while age was more important in women. At the same time, the shape of the lower pelvic canal strongly influences the shape of the pelvic muscles in both sexes. In contradiction with epidemiological studies, we did not find a clear association between the pelvic floor shape and the number of live births in women, suggesting that age plays a more important role in the muscle tone and the resultant shape of the female pelvic floor tissues.