Why is human childbirth so difficult? Obstetrics and the evolution of labor

Join us for a conversation with Philipp Mitteroecker and Barbara Fischer, of the Dept. of Evolutionary Biology at the University of Vienna. The incidence of obstructed labor in humans is strikingly high, in the range of 3-6% worldwide, mostly resulting from the disproportion of the mother's pelvic dimensions and the newborn's head. Mortality and morbidity due to this disproportion imposes a strong (and partly persisting) selection pressure. Why has natural selection not led to a wider female birth canal and reduced obstructed labor?

We present a model that explains the high rate of obstructed labor. Drawing from epidemiology and evolutionary quantitative genetics, the model allows for an estimation of the strength of selection on neonatal and maternal dimensions. We show how moderate directional selection suffices to account for the high rates of cephalopelvic disproportion and discuss why selection is unable to reduce these rates. The model predicts a considerable evolutionary response of pelvic and/or neonatal dimensions resulting from the regular use of Caesarean sections, and explains the intergenerational "inheritance" of Caesarean delivery. We show how environmental, economic, and demographic transitions contribute to the global rates of Caesarean section. This illustrates the importance of evolutionary theory to understand biosocial and epidemiological change in modern societies.

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