<p>The first months after a baby’s birth encompass the most rapid period of postnatal change in the human lifespan, but temporally precise trajectories of white matter maturation in this period remain largely uncharted. Using densely sampled diffusion tensor images from predominantly term-born infants, we measured non-linear growth and growth rate trajectories of diffusion tensor imaging metrics from birth to 6 months. Overall white matter growth rates at birth were approximately 6 times faster than at 6 months, and less mature tracts developed the fastest. When matched on chronological age, shorter gestation infants had less mature white matter at birth but faster growth rates than their longer gestation peers; however, growth trajectories were highly similar when corrected for gestational age. These trajectories provide novel insight into the complex and dynamic processes underlying white matter development during the first six postnatal months, which can inform the study of neurodevelopmental disorders beginning in infancy.</p>