Gestational Week 20 as the Biomechanical Inflection Point of Retroperitoneal Fascial Lamination: A Mechanobiological Model Integrating Geometric Scaling and Evolutionary Front-Loading

The embryological basis for the lamination of the retroperitoneal fascia has long remained an anatomical paradox. Classical peritoneal fusion theories cannot account for either the highly organized multilaminar architecture of the mature fascia or the striking temporal lag between early visceral fixation (gestational weeks 9–18) and the abrupt, synchronized emergence of definitive fascial laminae around week 20. Integrating recent advances in fetal biomechanics, we propose that this developmental lag reflects a system-level mechanical transition driven by the geometric constraints of scaling and the evolutionary demands of obligate bipedalism.