Why is childbirth so difficult in humans?

Compared to the other great apes, birth is a remarkably painful and difficult process in humans. The difficulty stems from the tight fit between the fetal head and shoulders and the maternal pelvis. It is a puzzle why evolution has not resolved this tight fit despite the high associated mortality and morbidity for both mother and child. The central question of this project is an important unanswered question in anthropology and evolutionary medicine: Why could a bir th process that causes such high mortality evolve, and why does it still persist? A selective force has kept humans from evolving wider pelvises, but what causes this selection remains unknown. In this project a new hypothesis will be tested, proposing pelvic floor function as an explanation. The pelvic floor is a muscular diaphragm, that carries the abdominal organs, similar to a hammock. Only in humans the pelvic floor has this function because we are walking upright. A wider pelvis might increase the strain on the pelvic floor beyond its capacity. This might lead to frequent pelvic floor dysfunction, including urinal and anal incontinence as well as pelvic organ prolapse. Therefore, the capacity of the pelvic floor migh t be the reason why wider pelvises did not evolve, and why birth is so difficult. Understanding how the traits of mother and fetus have been affected by selection is of importance for evolutionary biology and anthropology, but it also has immediate implications for obstetric practice. Today, disproportion between pelvis and fetus can be resolved by Cesarean section where access to medical care is available. The findings from the evolutionary research in this project will subsequently be applied to develop a novel method for obstetrics to better predict feto-pelvic disproportion. This research will inform decision-making regarding the preferred mode of birth: Cesarean section or vaginal birth. To achieve these objectives, a large-scale study of 3D pelvic morphology will be conducted. Imaging data of human bodies will be integrated with genetic data, birth-associated data, and epidemiological data, and state-of-the art 3D morphometrics, multivariate statistics, and machine learning methods will be used to analyze these data.