Cognitive outcome varies in adolescents born preterm, depending on gestational age, intrauterine growth and neonatal complications

Epidemiological studies based on Scandinavian national registers have shown that adults born preterm tend to attain lower than average education and income and to have a higher degree of welfare support 1. Numerous follow-up studies in school-aged children have reported lower cognitive performance and more learning problems in preterm children 2,3, even among those born moderately preterm 4. Population-based longitudinal studies have shown that cognitive level in childhood, in particular with respect to executive functions, is related to social outcome in adulthood, as reflected by indices of health, wealth and public safety 5. It is reasonable to assume that a less successful social outcome in adults born preterm may be partially attributed to cognitive deficits. Several factors are known to contribute to suboptimal cognitive development after preterm birth. The lower the gestational age at birth, the higher the risk of subsequent cognitive deficits and learning difficulties 6. The effect is partially mediated by neonatal complications, such as intraventricular haemorrhage, white matter disease, chronic lung disease and retinopathy of prematurity. These complications have been related to distinct neurodevelopmental disorders, such as cerebral palsy, and also to poor cognitive development in the absence of such disorders 7,8. Furthermore, large cohort studies 9,10 have shown that children born small for gestational age have learning problems and inferior school outcome, indicating that intrauterine growth restriction has a long-term negative impact on cognitive development. Longitudinal studies of children born preterm indicate that the effect of preterm birth varies with age 11. For some children, the negative influence may become more obvious over time when increasing demands are put on the child, as indicated by higher needs for special education and lower school achievement 12. However, there are few follow-up studies on adolescents and young adults 10,13–15 and our knowledge about long-term cognitive outcomes after preterm birth is therefore still limited. The study was conducted as part of the Stockholm Neonatal Project, a longitudinal population-based study of children born preterm in 1988–1993, with a very low birthweight of <1500 g who had been recruited at birth and followed prospectively 16. During the initial recruitment process, all children who met these criteria and were born in Karolinska Hospital or Lowenstromska Hospital in Stockholm were invited to take part. In addition, all children from the entire county of Stockholm with a birthweight of 1000 g or less who were in need of neonatal intensive care at Karolinska Hospital received an invitation. In this study, all preterm children (n = 182) and matched controls born at term (n = 125), who participated in a previous follow-up assessment at the age of 5.5, were invited to participate in a psychological assessment at the age of 18. At 5.5 years of age, the preterm group had shown significantly lower results than the controls in visuo-motor, cognitive and executive function tests and their executive deficits were also significant when they were controlled for their intelligence quotient (IQ) 17. Low gestational age and intrauterine growth restriction late in pregnancy were associated with worse outcome, as were perinatal medical complications, particularly severe retinopathy of prematurity 7. The aim of this study was to investigate cognitive outcome in late adolescence, to determine whether the pattern of cognitive deficits noted in the follow-up at the age of 5.5 had diminished or remained stable over time and if perinatal factors would still predict outcome in late adolescence.