How do infant brains fold? Sulcal deepening is linked to development of sulcal span, thickness, curvature, and...

Cortical folding begins in utero as sulci emerge and continues postnatally as sulci deepen. However, the timeline and mechanisms of postnatal sulcal development remain poorly understood. Using structural and quantitative magnetic resonance imaging from 79 infant sessions spanning birth to one year, we longitudinally examined macroanatomical parameters and microstructural tissue density (longitudinal relaxation rate, [R₁]) across sulci that emerge in utero between 16-31 gestational weeks. Here we show that early emerging sulci are deeper at birth and deepen more slowly postnatally than later emerging sulci. Across the first year, sulci become wider (42%), thicker (21%), denser with tissue (33%), and less concave (14%). Mean depth is predicted by a weighted combination of sulcal span, thickness, curvature, and microstructure, with differential weights across sulci. Fine-grained analyses of local depth along the sulcus further reveal that sulcal fundi differ from sulcal walls: deeper portions show higher curvature and greater tissue density. These data show that postnatal sulcal deepening is nonuniform and reflect coordinated anatomical and microstructural processes anchored to the timing of sulcal emergence in utero. By establishing normative trajectories of postnatal sulcal development, our findings inform theories of cortical folding and offer a framework for characterizing deviations from typical development.