Cohort Profile: Multimorbidity in Children and Youth Across the Life-course (MY LIFE) Study.

Objective: This manuscript serves to provide an overview of the methods of the Multimorbidity in Children and Youth across the Life-course (MY LIFE) study, profile sample characteristics of the cohort, and provide baseline estimates of multimorbidity to foster collaboration with clinical and research colleagues across Canada.

Method: MY LIFE is comprised of 263 children (2-16 years) with a physical illness recruited from McMaster Children's Hospital, their primary caregiving parent, and their closest-aged sibling. Participants are followed with data collection at recruitment, 6, 12, and 24 months which includes structured interviews, self-reported measures, and biological samples and occur in a private research office or at participants' homes. Post-COVID-19, data collection transitioned to mail and telephone surveys.

Results: At recruitment, children were 9.4 (4.2) years of age and 52.7% were male. The mean duration of their physical illness was 4.5 (4.1) years; 25% represent incident cases (duration <1 year). Most (69.7%) had healthy body weight and intelligence in the average range (73.5%). Overall, 38.2% of children screened positive for ≥1 mental illness according to parent report (24.8% screened positive based on child self-report). Compared to 2016 Census data, the MY LIFE cohort overrepresents families of higher socioeconomic status.

Conclusions: Multimorbidity is common among children and these baseline data will serve to measure relative changes in the mental health of children with physical illness over time. MY LIFE will provide new information for understanding multimorbidity among children, though underrepresentation of lower socioeconomic families may have implications for the generalizability of findings.