Background Retrospective parental reports have often been used to identify the early characteristics of children later diagnosed with a developmental disorder.Method We applied this methodology to document 13 parents' initial concerns about the development of their 17 children later diagnosed with fragile X syndrome (FXS). Parents were additionally asked about when they noticed the emergence of behavioural signs related to FXS.Results More than half of the parents reported initial concerns prior to the child's first birthday, and in most cases it was atypical motor behaviours that caused the first concerns. Behavioural signs related to the FXS phenotype were also reported to be perceptible in the first year of the child's life.Conclusions Due to limitations of retrospective parental questionnaires, we suggest that other methodologies, such as home video analysis, are needed to complement our understanding of the pathways of developmental disorders with late clinical onsets.