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.
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