**5. Conclusions**

The results of parental data from this national age cohort of children with Down syndrome within a narrow age range indicate a significantly higher occurrence of difficulties with speech fluency compared to typically developing children of the same non-verbal mental age level. A significant association between difficulties with speech fluency and the level of language skills was discovered and should be taken into consideration when planning treatment for children with Down syndrome.

To date, limited research results on interventions and treatment of difficulties with fluency or fluency disorders for children with Down syndrome exist, and no effect study (e.g., a randomized controlled trial) is known to the authors. Until we know more about what constitutes effective treatment for this group of children, the large co-occurrence between difficulties with fluency and low language skills in children with Down syndrome supports a need for speech and language therapy that aims to simultaneously improve the child's language development and speech fluency. Speech-language pathologists also have a responsibility for treating the complex communication disorder of this group of children. To tailor the treatment to research-based knowledge, future effect studies should be designed, especially for children with Down syndrome; these studies should control for language level to investigate the potential effects of fluency treatment on language development.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/brainsci11060704/s1, Table S1. Correlation matrix (N = 41).

**Author Contributions:** Conceptualization, K.-A.B.N., E.N., H.H., J.S.Y.; methodology, K.-A.B.N., E.N., H.H.; formal analysis, E.N., K.-A.B.N.; investigation, K.-A.B.N.; resources, K.-A.B.N.; data curation, K.-A.B.N., E.N.; writing—original draft preparation/introduction K.-A.B.N., H.H., J.S.Y., E.N./method K.-A.B.N., H.H., E.N./results. E.N./discussion K.-A.B.N., E.N., H.H., J.S.Y.; writing review and editing, K.-A.B.N., E.N., H.H., J.S.Y.; visualization, K.-A.B.N. and E.N.; project administration, K.-A.B.N.; funding acquisition, K.-A.B.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Research Council of Norway, grant number 238030.

**Institutional Review Board Statement:** Regional Committee for Medical and Health Research Ethics Sør-øst. IRB ID: 19732.

**Informed Consent Statement:** Informed consent was obtained from all families involved in the study.

**Data Availability Statement:** The data are available in the services for sensitive data at the University of Oslo and can be obtained by contacting the first author.

**Acknowledgments:** We would like to thank the participating children and parents, schools, and research assistants as well as the funder of the study: grant number 238030 from the Research Council of Norway.

**Conflicts of Interest:** The authors declare no conflict of interest.
