TRACKING FETAL CORTICAL MORPHOLOGY IN THE 2ND AND 3RD TRIMESTERS REVEALS A SHARP ONSET OF GYRIFICATION NEAR GW27, BUT A GRADUAL EMERGENCE OF FRACTAL SCALING

We analyzed structural MRI from 102 non‑pathological fetal specimens (13–40 GW), including postmortem T1w in‑skull scans, in‑utero T2w scans, and post‑term T2w scans. We developed pipelines to segment the cortical plate/cortex and extract pial and white‑matter surfaces using manual and automatic tools, and we quantified the evolution of cortical volume, thickness, areas, and other relevant morphometrics. We show a sharp transition from a nearly lissencephalic regime to increasing gyrification at GW27, visible in both the gyrification index and in K, a folding‑based structural descriptor derived from a universal scaling law observed in healthy adult brains across mammalian species and human cohorts. Interestingly, there are also hints of a second, transient gyrification mechanism operating early in the first trimester. Cortical‑volume growth rate increases gradually between GW27 and GW32, marking a transition between two linear regimes, while cortical thickness increases at roughly the same rate throughout. In contrast, a multiscale analysis of folding shows that the developing cortex, unlike its mature form, is not fractal, as different length scales transition from a lissencephalic to a gyrencephalic regime at different times. Altogether, these results show that cortical‑morphological development is not a homogeneous process but instead unfolds through a complex concatenation of distinct regimes as it approaches its self‑similar, universally scaled adult form.