Background: The literature on brain imaging in premature infants, is mostly made up of studies that evaluate neonates, yet the most dynamic time of brain development happens from birth to one year of age. This study was designed to obtain quantitative brain measures from Magnetic Resonance Imaging (MRI) scans of infants born prematurely at 12 months of age. Methods: The subject group was designed to capture a wide range of gestational age (GA) from premature to full term infants. An age-specific atlas generated quantitative brain measures. A regression model was used to predict effects of gestational age, sex, on brain measures. Results: There was a primary effect of sex on: 1) intracranial volume (ICV), males > females; 2) proportional cerebral cortical gray matter (females > males) and 3) cerebral white matter (males> females). GA predicted cerebral volume and cerebral spinal fluid (CSF). GA also predicted cortical gray matter in a sex specific manner with GA having a significant effect on cortical volume in the males, but not in females. Conclusions and Relevance: Sex differences in brain structure are large early in life. GA had sex specific effects highlighting the importance evaluating sex effects in neurodevelopmental outcomes of premature infants.