Brain folding as a Fourier series yields a developmental clock

The human fetal cortex begins as a smooth spheroid and progressively folds into a stereotypical sulcal pattern. I show that the spherical-harmonic expansion - the closed-surface generalisation of Fourier's series - reproduces this developmental sequence in reverse on the group-average atlas: truncating the fully formed cortex at low maximum harmonic degree L_max recovers a younger fetal brain; progressively higher L_max recovers the primary, secondary and tertiary folds in the order in which they emerge. From this I derive a closed-form 23-dimensional log-fractional spectral descriptor. On atlas templates (17 Harvard CRL + 16 dHCP weekly templates) it predicts gestational age at MAE 0.13 and 0.38 wk across cohorts after a single supervised linear alignment, and 1.07 and 1.51 wk without. On 28 Yale-Wayne State longitudinal pairs it advances predicted GA in every subject, without cohort matching. On 80 FeTA scans (31 neurotypical, 49 pathological) off-trajectory distance discriminates pathology at AUC 0.80, and unsupervised clustering yields three internally stable, biometrically concordant subgroups - an exploratory finding requiring external validation. Individual-scan gestational-age accuracy is not the descriptor's competitive use (MAE 2.25 wk on FeTA-NT); its individual-scan value is the shape of the residual around the normative trajectory. The framework separates two spatial-scale regimes of folding onto distinguishable spectral bands and is complementary to existing measures of fetal cortical maturation.