Objective To explore relationships of early postnatal cranial ultrasonography (cUS) linear measures of brain size and brain growth with neurodevelopment at 2 years in infants born <30 weeks’ gestational age (GA) and free of major brain injury.

Design Prospective observational cohort study.

Setting Tertiary neonatal intensive care unit.

Patients 139 infants born <30 weeks’ GA, free of major brain injury on neonatal cUS and without congenital or chromosomal anomalies known to affect neurodevelopment.

Intervention Linear measures of brain tissue and fluid spaces made from cUS at 1-week, 1-month and 2-months’ postnatal age.

Main outcome measures Cognitive, language and motor scores on the Bayley Scales of Infant and Toddler Development, third edition at 2 years’ corrected age.

Results 313 scans were evaluated from the 131 children who were assessed at 2 years. Larger measures of the corpus callosum at 1 week, 1 month and 2 months, cerebellum and vermis at 2 months and faster positive growth of the cerebellum and vermis between 1 month and 2 months, were related to higher cognitive and language scores at 2 years. No relation between tissue measures and motor scores was found. Larger measures, and faster rate of increase, of fluid spaces within the first weeks after birth were related to better cognitive, language and motor outcomes at 2 years.

Conclusions Early postnatal cUS linear measures of brain tissue were related to cognitive and language development at 2 years in infants born <30 weeks’ GA without major brain injury. Relationships between cUS linear measures of fluid spaces in the early postnatal period and later neurodevelopment warrant further exploration.