RESUMO
OBJECTIVE: We aimed to determine the timing for assessing birth status of the developing brain (i.e. brain maturity at birth) by exploring the postnatal age-related changes in neonatal brain white matter (WM). METHODS: The institutional review board approved this study and all informed parental consents were obtained. 133 neonates (gestational age, 30-42 weeks) without abnormalities on MRI were studied with regard to WM development by diffusion tensor imaging-derived fractional anisotropy (FA). Tract-based spatial statistics (TBSS), locally-weighted scatterplot smoothing (LOESS) and piecewise linear-fitting were used to investigate the relationship between FA and postnatal age. FA along corticospinal tract (CST), optic radiation (OR), auditory radiation (AR) and thalamus-primary somatosensory cortex (thal-PSC) were extracted by automated fibre-tract quantification; their differences and associations with neonatal neurobehavioural scores at various postnatal age ranges were analysed by Wilcoxon's rank-sum test and Pearson's correlation. RESULTS: Using TBSS, postnatal age (days 1-28) positively correlated with FA in multiple WMs, including CST, OR, AR and thal-PSC (p<0.05). On the other hand, when narrowing the postnatal age window to days 1-14, no significant correlation was found, suggesting a biphasic WM development. LOESS and piecewise linear-fitting indicated that FA increased mildly before day 14 and its growth accelerated thereafter. Both FA and correlations with neurobehavioural scores in postnatal age range 2 (days 15-28) were significantly higher than in range 1 (days 1-14) (FA comparison: p<0.05; maximal correlation-coefficient: 0.693 vs. 0.169). CONCLUSION: Brain WM development during the neonatal stage includes two phases, i.e. a close-to-birth period within the first 14 days and a following accelerated maturation period. Therefore, evaluations of birth status should preferably be performed during the first period. KEY POINTS: ⢠Brain white matter development within the first two postnatal weeks resembles a close-to-birth maturation. ⢠Brain white matter development in the audio-visual, sensorimotor regions accelerates after two postnatal weeks. ⢠Postnatal age-related effects should be considered in comparing preterm and term neonates.