The Impact of Norms on the Outcome of Children Born Very-Preterm when Using the Bayley-III: Differences between US and German Norms

Background: The Bayley Scales of Infant and Toddler Development, third edition (Bayley-III) are widely used to assess the development of children born preterm. However, it is still unclear whether US norms are adequate for use in other populations. In 2014, the German version of the Bayley-III with German norms was published. Objectives: We aimed to compare the performance of very-preterm infants at one, two, and three years of age using the German versus US norms of the Bayley-III. Methods: All children born prior to 32 weeks of gestational age during 2012 and 2015 and taking part in the follow-up program of the Medical University of Vienna were assessed and scored according to both German and US norms by two expert clinical psychologists at the age of one, two, and three years of age. Results: In total, 843 tests were obtained from 450 children. When looking at scaled scores for cognitive, language, and motor outcomes, preterm children achieved significantly lower scores in nearly all subtests with the German as compared to the US norms. In addition, the proportion of impaired children scoring < 1 standard deviation (SD) and < 2 SD below the norm was significantly higher in nearly all subtests when using the German compared to the US norms (p < 0.05). Conclusions: Applying German and US norms of the Bayley-III leads to different outcomes in very-preterm children, and these differences are statistically significant and clinically relevant. Thus, comparisons of outcomes of very-preterm children using culturally specific norms are problematic, and these findings have to be considered.