Over the edge: Empirical evidence for the cliff-edge model of obstetric selection

The cliff-edge model of obstetric selection maintains that larger neonates and smaller birth canals confer a positive selective advantage until labor becomes obstructed and vaginal delivery is no longer possible, eliciting an abrupt reduction in fitness. The model postulates that even weak directional selection, on either the maternal pelvis or neonatal size, is sufficient to explain the observed incidences of fetopelvic disproportion. The regular and safe use of Caesarean sections has alleviated obstetric constraints and is expected to have increased fetopelvic disproportion rates. Using a historic Swiss dataset (babies born from 1896 to 1939) and a modern French dataset (babies born from 2011 to 2013), we calculated a composite variable, $\hat{D}$ , capturing the fetopelvic fit to test the assumption that birth interventions can shift the distribution of births at risk for fetopelvic disproportion. Our 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. The percentage of births with $\hat{D}>0$ , predictive of fetopelvic disproportion risk, increased from 0.05% in the historic dataset to 14.6% in the modern dataset. Within the historic dataset, head circumference was a significant predictor of Caesarean section, whereas birthweight was not. By quantifying a historic-to-modern shift in $\hat{D}$ , our study applies the cliff-edge model of obstetric selection to real-world data and illustrates how medical interventions can alter the evolutionary dynamics of parturition and contribute to the persistence and amplification of susceptibility to fetopelvic disproportion.