**5. Conclusions**

Between 2010 and 2017, using national database registries, Canadian women 40–49 years old were diagnosed with significantly fewer stage I and more stages II and III breast cancers than women 50–59 years old. In the same period, women 40–49 years old in screening jurisdictions were diagnosed with significantly fewer stages II, III, and IV BC than those living in the comparator provinces. Screening policies for women 40–49 also had an impact on women 50–59 years old who had significantly lower rates of stages II and III BC in the screening vs. the comparator jurisdictions.

**Author Contributions:** Conceptualisation, A.N.W., J.M.S., L.F.E. and J.-M.B.; methodology, J.-M.B., L.F.E., A.N.W. and J.M.S.; software, J.-M.B., L.F.E. and A.N.W.; validation, A.N.W., J.M.S., L.F.E. and J.-M.B.; formal analysis, J.-M.B. and L.F.E.; investigation, J.-M.B., L.F.E., A.N.W., J.M.S., N.I. and M.A.K.; resources, J.M-.B., L.F.E., A.N.W., J.M.S., N.I. and M.A.K.; data curation, A.N.W., L.F.E., J.-M.B. and J.M.S.; writing—original draft preparation, A.N.W. and J.M.S.; writing—review and editing, J.-M.B., L.F.E., A.N.W., J.M.S., N.I. and M.A.K.; visualisation, A.N.W., J.M.S., L.F.E. and J.-M.B.; supervision, J.-M.B., L.F.E., A.N.W. and J.M.S.; project administration, L.F.E. and J.-M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** This study was a secondary analysis of nationally deidentified data collected by Statistics Canada, and as such, ethics approval was not required.

**Informed Consent Statement:** Patient consent was waived this study was a secondary analysis of nationally de-identified data collected by Statistics Canada.

**Data Availability Statement:** Aggregate data on cancer incidence counts and rates by stage are publicly available from Tables 13-10-0761-01 and 13-10-0762-01, patient- and tumor-level microdata files are confidential and can only be accessed through Statistics Canada's Research Data Centre Program.

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