Objective: Our goal was to investigate the association between failure to thrive (defined as weight faltering in the first 9 months of life) and IQ levels 8 years later.
Methods: Weight gain (conditional on initial weight) from birth to 8 weeks, 8 weeks to 9 months, and birth to 9 months was measured on term infants from the Avon Longitudinal Study of Parents and Children. Cases of weight faltering were defined as those infants with a conditional weight gain below the 5th centile who were compared with the rest of the cohort as the control group. At the age of 8 years, 5771 infants born at term with no major congenital abnormalities had IQ measured by using the Wechsler Intelligence Scale for Children, Third Revision.
Results: Mean (SD) IQ scores were 104.7 (16.3) (total), 107.6 (16.5) (verbal), and 100.2 (16.9) (performance). Children whose weight faltered from birth to 9 months had a total IQ that was significantly lower by an average of -2.71 points at 8 years, equivalent to 0.17 SD. Weight gain from birth to 8 weeks had a positive linear association with child IQ at 8 years. This remained significant in a multivariate regression despite controlling for correlates of both infant growth and child IQ; 1 SD of weight gain was associated with a difference of 0.84 points in the total IQ score. In contrast to early weight faltering, weight gain from 8 weeks to 9 months was not related to IQ at 8 years.
Conclusions: Failure to thrive in infancy was associated with persisting deficits in IQ at 8 years; the critical period for growth faltering was birth to 8 weeks. The relationship between infant growth from birth to 8 weeks and later intellectual development was approximately linear over the whole range of weight velocities.