Cognitive outcome at 24 months is more predictive than at 18 months for IQ at 8–9 years in extremely low birth weight children

BackgroundIt is unclear whether developmental assessment later or earlier in childhood is the better predictor of intelligence at 8 years of age. This is an important distinction as many clinical trials assess their final outcomes only in early childhood, assuming the results are valid for later childhood cognitive functioning.AimsTo compare the ability of developmental assessment at 18 months with 24 months in predicting general intellectual functioning at 8–9 years of age in extremely low birth weight (ELBW, birthweight < 1000 g) children.Study designCohort study.Subjects58 ELBW survivors born during 1997 at the Royal Women's Hospital, Melbourne, Australia.Outcome measuresCognitive assessments at each of 18 months, 24 months (Mental Developmental Index [MDI]) and 8–9 years (Full Scale IQ) of age, corrected for prematurity were compared by regression analysis and by the κ statistic (agreement beyond chance).ResultsBoth the 18-month and the 24-month MDI were significantly predictive of Full Scale IQ at 8–9 years, but more so for the 24-month MDI, with 38% of variance explained compared with 34% of variance explained by the 18-month MDI. The 24-month MDI, expressed as categories of severe, moderate, mild or no developmental delay, was more predictive of categories of severe, moderate, mild or no intellectual impairment at 8–9 years (weighted κ = 0.43, P < 0.001) than was the 18-month MDI (weighted κ = 0.35, P = 0.001).ConclusionsCognitive assessment at 24 months is superior to cognitive assessment at 18 months in predicting IQ and intellectual impairment at 8–9 years of age in ELBW children.