Underestimation of developmental delay by the new Bayley-III Scale.

Objective: To assess the ability of the third edition of the Bayley Scales of Infant and Toddler Development (Bayley-III) to detect developmental delay in 2-year-old children who were extremely preterm and those carried to term.

Design: Prospective cohort study.

Setting: The state of Victoria, Australia.

Participants: Subjects were consecutive surviving children who were born either at less than 28 weeks' gestational age (extremely preterm) or with less than 1000 g birth weight (extremely low-birth-weight; n = 221) in the state of Victoria, Australia, in 2005 and randomly selected controls who were both carried to term and of normal birth weight (n = 220).

Main Outcome Measure: Children were assessed by psychologists blinded to knowledge of group at 2 years of age, corrected for prematurity with the new Bayley-III scale.

Results: Follow-up rates of both cohorts were high (>92%). Mean values for all composite and subtest scores for the extremely preterm/extremely low-birth-weight group were significantly below those of the control group (P < .001), with the magnitude of all group differences being in excess of two-thirds SD. Mean values for the extremely preterm/extremely low-birth-weight group approached the normative mean, but in contrast, the mean values for the control group were higher than expected, with composite scores being between 0.55 and 1.23 SD above the normative mean. Proportions of children with developmental delay were grossly underestimated using the reference values, but were within the expected range when computed relative to the mean (standard deviation) for the controls.

Conclusion: The Bayley-III scale seriously underestimates developmental delay in 2-year-old Australian children.