Many individual factors, such as early communicative skills, could play a role in explaining later linguistic outcomes. The detection of predictive variables is fundamental to identifying early the children who need intervention. The present study focuses on children with sex chromosome trisomies (SCTs), genetic conditions with an increased risk of developing language delays or impairments. The aims are to analyse their communicative skills at 18 months of age, and identify significant predictors of their later vocabulary size. Participants were 76 18-month-old children (38 with SCTs, and 38 typically-developing (TD) children). Their communicative skills were assessed during a parent–child play session, and parents filled in a report on their vocabulary development at 18 and 24 months. Children with SCTs showed significantly poorer linguistic skills at 18 months in both preverbal (babbling and gestures) and verbal abilities. A high percentage (nearly 70%) of toddlers with SCTs were late-talking children at 24 months, and those toddlers showed a lower frequency of babbling utterances at 18 months. Early lexical skills, children’s developmental quotient, and being part of the group of toddlers with SCTs were significant predictors of children’s vocabulary size six months later. These variables should be considered when assessing the linguistic competence of a child with SCTs to detect possible early risk factors of future language impairment.

Participants included in this study were 87 children ranging in age from 18 to 26 months (M 5 23.59; SD 5 e198 Children's Behavior in Sex Chromosome Trisomies