Children’s language skills.

Children’s language development was assessed at TP 3 (14~23 months) and TP 5 (20~25 months) using the language scale of the Bayley Scales of Infant and Toddler Development – Third Edition²¹. The language scale consists of two subtests, measuring children’s receptive communication (49 items) and expressive communication (48 items) skills; each item is scored zero or one. The first item to administer is determined based on the child’s age, and test administration stops when the child misses five consecutive items. In addition, the cognitive scale of the Bayley was also administered. The cognitive scale is comprised of 91 items that assess children’s sensorimotor development, exploration and manipulation, object relatedness, concept formation and memory. Although our primary interest in this study is children’s language development, we include the Bayley cognitive outcomes as a second index of early brain development among our sample.

Based on the sum of correct responses, scaled scores (normed to a population mean of 10 and standard deviation of 3) are created for each scale. We used these scaled scores as child outcome variables for ease of interpretation. High reliability was reported for both scales per the test manual (language = 0.93, cognitive = 0.91).