Objective: To investigate the stability of development in general intelligence (IQ) and executive functions (EF), and to identify predictors of cognitive development from preschool age through adolescence, in a cohort of preterm and term-born children.
Participants and Methods: As part of the longitudinal Stockholm Neonatal Project, 134 adolescents born preterm with very low birth weight (< 1500g) and 94 matched controls born at term, participated in follow-up studies at ages 5 ½ and 18 years. On both occasions, the participants were assessed with an age appropriate Wechsler IQ test and tests of executive functions (EF). Stability in cognitive outcome was tested with Pearson correlations. Stepwise regression analyses were used to investigate how cognitive outcome at age 18 was predicted by EF at age 5 ½, parental education, gender, medical risk, and birth weight ratio.
Results: IQ was quite stable form preschool age through adolescence (r =.78 in whole group, r = .84 in preterm and r= .61 in controls), as were EF (r = .65 in whole group, r = .64 in preterm and r = .53 in controls). In controls, EF at age 5 ½ and parental education predicted IQ (R² = .205) and EF at 18 years (R² = .249). In the preterm group, EF at age 5 ½, parental education, medical risks and birth weight ratio predicted IQ 18 years (R² = .508), and EF at age 5 ½ and medical risks predicted EF at18 years (R² = .432).
Conclusions: Stability in cognitive outcomes from preschool through adolescence was high, particularly in the preterm group. EF at 5 ½ strongly predicted cognitive and executive functions at 18 years, in both groups. Over the same period, parental education had a continued positive effect on cognitive development. Among preterm children, perinatal medical risks predicted a less favorable continued cognitive development, especially in EF, and a low birth-weight ratio had a negative impact on general intelligence.