Consequences of using chronological age versus corrected age when testing cognitive and motor development in infancy and intelligence quotient at school age for children born preterm

Background: Children born preterm (<37 weeks' gestation) have an increased risk of poor neurodevelopment, including lower intelligence quotient (IQ) scores compared with their term-born counterparts. 

Objective: To explore the differences in psychometric scores for cognition and motor skills when they are age-standardized according to chronological age instead of corrected age for children born preterm. 

Methods: We assessed = 554 children born <33 weeks' gestation with the Bayley Scales of Infant Development, 2nd edition (mental and motor scores) at 18 months and the Weschler Abbreviated Scale of Intelligence (IQ score) at seven years of age. Scores were standardized according to chronological age and corrected age and differences between mean chronological and corrected scores were compared, along with the proportion of children whose scores could be classified as impaired. 

Results: When scores were standardized according to chronological age instead of corrected age there was a large significant difference of 17.3 points on the mental scale (79.5 vs. 96.8, respectively) and 11.8 points on the motor scale (84.8 vs. 96.6, respectively) at 18 months. By seven years, the difference in IQ scores remained, although of a smaller magnitude at 1.9 points between mean chronological and corrected age scoring (97.2 vs. 99.1, respectively). 

Conclusion: Consistent with previous literature, outcome assessments for preterm infants consistently differed according to use of chronological or corrected age to standardized scores. Cognitive scores were impacted more severely than motor scores, and differences were more substantial in early childhood than later in childhood. For clinical purposes, correction for preterm birth is only likely to have an impact during early childhood, however assessments for research purposes should continue to correct into childhood to account for the persistent bias due to preterm birth.