Human brain R2* transitions across birth from the womb to early infancy.

Iron plays a vital role in early brain development, supporting critical processes such as myelination, dendritogenesis, and neurotransmitter synthesis. The perinatal period marks a crucial transition from the intrauterine to the extrauterine environment, requiring significant brain adaptation to new stimuli and metabolic demands. However, tight spatiotemporal resolution capturing the timing and sequence of brain iron changes surrounding this critical transition has yet to be achieved. Leveraging a longitudinal perinatal cohort with 147 multi-echo MRI scans spanning from 25 to 60 post-conceptual gestational weeks, we mapped brain iron growth trajectories with R2* estimation across fetal, newborn and neonatal periods. We also examined whether sex, gestational age at birth, and birth weight influence R2* developmental trajectories. We found that parietal and superior temporal regions predominately show linear growth trajectories throughout the perinatal period across birth, while the occipital cortex, the temporal pole, inferior temporal regions and a subset of frontal regions exhibit non-linear trends. For most of the non-linear trajectories, growth rates peak around 40 weeks, highlighting the critical window of birth transition for brain R2* change. These results provide the first longitudinal insights into R2* development across birth, uncovering distinct regional growth patterns that may align with different phases of neurodevelopment.