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Background and aimsThe most common neurologic impairment in children born extremely preterm is in cognitive function, but little is known about their cognitive development from infancy to early adulthood. The aim of the study was to determine the trajectory of cognitive test scores from infancy to adulthood in infants born extremely preterm compared to term-born individuals, and the impact of infant sex, maternal education, gestational age and neonatal brain injury on these trajectories.MethodsWe conducted a longitudinal analysis of cognitive trajectories in a prospective, population-based cohort of 315 infants born less than 26 completed weeks and 160 term-born classroom controls followed-up to 19 years. Participants were recruited from all 276 maternity units in the United Kingdom and the Republic of Ireland in 1995. The cognitive assessments used were the Bayley Scales of Infant Development – II (age 2.5), Kaufman Assessment Battery for Children (ages 6 and 11), and the Wechsler Abbreviated Scale of Intelligence-II (age 19). Assessments were administered individually by a highly qualified assessor, blinded to preterm birth status and previous medical history.ResultsThe mean cognitive scores of extremely preterm individuals over the period were on average 25.2 points below their term-born peers (95% CI: −27.8 to −22.6) and significantly lower at every assessment. Cognitive test scores were stable in both groups, though there was a small but significant increase of 0.5 IQ points per year in the extremely preterm group. Cognitive scores in term males and females did not differ significantly, but the scores of extremely preterm males were 8.8 points below those of extremely preterm females (95% CI: −13.6 to −4.0). Higher maternal education elevated scores in both groups by 3.2 points (95% CI: 0.8 to 5.7). Within the extremely preterm group, moderate to severe neonatal brain injury (mean difference: −10.9, 95% CI:−15.5 to −6.3) and gestational age less than 25 weeks (mean difference: −4.4, 95% CI:−8.4 to −0.4) also had an adverse impact on cognitive function.ConclusionsThere is no evidence that impaired cognitive function in extremely preterm individuals materially recovers or deteriorates from infancy through to 19 years. Estimates made in infancy and early childhood reflect early adult outcomes.