Impact of a matched term control group on interpretation of developmental performance in preterm infants.

One hundred twenty-four children who were born at 24 to 31 weeks' gestation and 124 term children matched in social background underwent serial developmental evaluations. The Bayley Mental Developmental Index at 6, 15, and 24 months and the McCarthy General Cognitive Index at 4 years were used to classify cognitive outcome for preterm children as normal (indices higher than 1 SD below the mean), mild-moderately delayed (indices between 1 and 2 SD below the mean), or severely delayed (indices > or = 2 SD below the mean). Classifications based on norms derived from the performance of the term control group were compared with those based on published standardized test scores. The control group had substantially higher mean (+/- SD) Bayley Mental Developmental Indices at 6 (111 +/- 11), 15 (114 +/- 13), and 24 months (115 +/- 21) than the published test mean (100 +/- 16). Consequently, significantly more preterm children were classified as normal when the Bayley test mean was used than when the performance of the control group was used to define the normal range (84% vs 52% at 6 months, 82% vs 49% at 15 months, and 70% vs 47% at 24 months). Severe cognitive delays were infrequent when defined by test mean (6% to 11%) but two to three times greater when the control group scores were used. In contrast, the control group had a mean McCarthy General Cognitive Index at 4 years (102 +/- 14) that was similar to the published test mean (100 +/- 16).(ABSTRACT TRUNCATED AT 250 WORDS)