A recent paper in The Anatomical Record, co-authored by KLI group leader Barbara Fischer and KLI president Philipp Mitteroecker, provides strong empirical support for the cliff-edge model of obstetric selection and shows how modern obstetrics is reshaping the relationship between maternal pelvic anatomy and fetal head size.
The study compares historic Swiss birth records (1896–1939) with contemporary French data (2011–2013) to quantify changes in fetopelvic fit across roughly four generations. The authors calculated a composite variable D?, capturing the fetopelvic fit, to test how birth interventions can shift the distribution of births at risk for fetopelvic disproportion. Their findings support the prediction that the relaxation of stabilizing selection following the widespread adoption of routine Caesarean sections facilitates the continued accumulation of susceptibility to fetopelvic disproportion. They illustrate how medical interventions can alter the evolutionary dynamics of parturition and contribute to the persistence and amplification of susceptibility to fetopelvic disproportion.

Publication:
Watson, L. M., Frémondière, P., Staub, K., Mitteroecker, P., Fischer, B., Haeusler, M., & Webb, N. M. (2026). Over the edge: Empirical evidence for the cliff-edge model of obstetric selection. The Anatomical Record, 1–15. https://doi.org/10.1002/ar.70159