SignificanceCharting how the brain develops is key to understanding abnormal brain changes in common psychiatric and neurological disorders. Pooling brain scans from large cohorts of individuals at a specific point in time—i.e., cross-sectionally—has allowed researchers to indirectly infer dynamic brain changes across the human lifespan. However, it is unknown whether this inference is accurate—do brain growth charts estimated from cross-sectional snapshots accurately mirror true brain changes observed in the same individuals scanned at multiple timepoints? Here, we demonstrate that brain charts inferred from cross-sectional data underestimate brain changes directly observed in longitudinal data. As we endeavor to accurately map human brain development, we must also incorporate longitudinal measurements of the brain.