Marco conceptual.

Three important limitations of the present study will be discussed. The sample size is relatively low, with 39 children with DS at T1 and 20 children at T2. Although the recruitment of participants with DS is not easy and small sample sizes are common in longitudinal studies with children with DS, this restriction limits the generalisability of the findings of this study. In small sample sizes wide variability in performance, as seen in the wide range of vocabulary scores in the present study, may limit conclusions about typical development in children with DS (Patterson, Rapsey & Glue, 2013). These differences are, however, found to be common in their vocabulary development (Galeote et al., 2008). Based on successive stepwise multilinear regressions, we constructed a structural relations model. The total model could not be assessed on goodness-of-fit in the present study, due to a lack of power given the small sample size and the differing N between T1 (N = 36) and T2 (N = 20). As such, the total model should be interpreted with caution. In a future study, our structural relations model of vocabulary development should be tested in a larger population with a Structural Equation Modeling (SEM) technique, both in children with DS as well as in a control group of typically developing children. SEM is a statistical approach to model means and covariances among multivariate data and allows for a simultaneous analysis of all variables in the model, visualized by a graphical path diagram. The present study does provide important insights in predictor variables of receptive and expressive vocabulary development of children with DS. Given that it is the first study to include a wide range of child- and environment-related factors in predicting development, results need confirmation in order to investigate the predictive role of all variables at once in both children with DS as well as typically developing children.

A second limitation may be that we used many parental questionnaires for several variables. There are some studies that questioned the accuracy of parental questionnaires, particularly in atypical groups such as DS, in which parents tend to over- or underestimate the abilities of their child (Roberts et al., 1998). However, we used instruments that have proven to be reliable or valid in children with DS, such as the Vineland Screener (Van Duijn et al., 2009) and the N-CDI (Deckers et al., 2016). Furthermore, it has often been stated that children with DS appear to have an ‘on-demand problem’, due to which skills demonstrated in spontaneous settings may be absent during assessment of that skill (Fidler &

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Nadler, 2007), indicating that results on standardized tests would not likely have resulted in more reliable measurements of variables included in the present study. Another limitation of the present study could be that we have not investigated motor skills in depth as indicated earlier. Advanced motor skills provide infants with more opportunities for experiencing the world (Iverson, 2010). Changes in posture, locomotion, and object-manipulation and being able to move through their environments makes children to experience objects in their environment in a new manner, possibly influencing how children develop their vocabulary (Behrens & Hauch, 2011). Children with slow motor development were found to have smaller vocabularies (Viholainen et al., 2002). Future studies would benefit from investigating the role of fine, gross and speech motor skills in vocabulary development in children with DS in more detail.

Given the broad range of the number of manual signs used by the children with DS in the present study, following Galeote et al. (2008), it is beneficial to explain to parents that sign instruction in the early stages of language and vocabulary development helps to improve initial communication of children with DS, and increases speech production (Millar, Light, & Schlosser, 2006; Schlosser & Wendt, 2008). It is also beneficial for parents and SLPs to keep using signs after the moment a child with DS starts to speak (Vandereet et al., 2011), for two reasons. The first reason for continuing the use of signs augmenting speech is that the visual-motor modality of signs may increase understanding of a message by the child, especially when auditory discrimination or auditory working memory are weak or impaired. Secondly, it is a clinical observation during the 1;6 years of the present study that parents of children who started to speak more, left out using signs themselves as well. Possibly as a result, we observed that some children used less signs overall, even for words they did not produce verbally, leading to a decrease in number of words expressed or a stagnation in vocabulary development. This trend should be investigated in more detail in future studies. Thirdly, the use of manual signs slows down the speed of speech production and shows an increase in intonation, prosody in the language input of the caregivers, which facilitates the speech and information processing in children with DS (Beukelman & Mirenda, 2013).

Conclusion

Within a longitudinal design, the present study showed several predictors for receptive and expressive vocabulary development in young children with DS. Predictors found in these children seem to resemble those also predicting vocabulary development in typically developing children, as found in other studies. Receptive vocabulary development was best predicted by the adaptive level of functioning, and early receptive vocabulary skills. Expressive vocabulary development was best predicted by the adaptive level of functioning, receptive vocabulary, maternal educational level, level of communicative intent of the child, attention skills and phonological/phonemic awareness. These results show that it is important for both the research and clinical field to focus on both child- and environment-related variables when looking at vocabulary development in children with DS. Following the model of holistic development (Fröhlich & Haupt, 2004) as described in the introduction, the vocabulary development of children with DS is interrelated with several other developmental domains, which should be accounted for in both research and clinical practice.

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