The illusion of catch-up growth in premature infants. Use of the growth index and age correction.

Growth data of 61 premature infants who were observed for 29 months were analyzed to determine if there is validity to the concept of catch-up growth. Weight, length, and head circumference were recorded at each visit. For each measurement, a growth index was calculated to facilitate comparison of infants of different ages and sexes. The age of a child at each visit was also corrected for prematurity by subtracting the number of weeks premature from the postnatal chronological age to generate a corrected age. The corrected age was then used to create a corrected growth index for all variables at each visit. Growth data evaluated in the traditional manner (without correction) suggested that catch-up growth occurred. However, growth data for corrected age more closely approximated the growth of full-term children. Graphic impressions were generally substantiated by Hotelling's T2 tests and factorial and one-way repeated-measures analyses of variance with Bonferroni adjustments. The corrected growth data of appropriate-for-gestational-age children approximated normal expected growth, whereas small-for-gestational-age children still demonstrated subnormal growth at 29 months of age, despite correction. In both appropriate-for-gestational-age and small-for-gestational-age children, catch-up growth appears to be a statistical illusion created by charting growth of premature infants using chronological age rather than the more appropriate corrected age. This finding has important implications for the follow-up of premature infants and may affect the diagnostic interpretation of failure to thrive.