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Background and Objective: Foramen magnum stenosis (FMS) is a common, serious complication of achondroplasia in infancy and associated with sudden infant death. The Achondroplasia Foramen Magnum Score (AFMS; 0-4) is used to classify the severity of stenosis to inform appropriate neurosurgical management. Infants with AFMS4 are referred for neurosurgery, while well children with AFMS3 undergo repeat MRI routinely after 12 months.As the natural history of children with AFMS3 is currently unclear, the objective was to review follow-up MRI scans of infants initially classified as AFMS3 to define more clearly the evolution of this degree of stenosis., Design: This retrospective cohort study, from two tertiary centres, included infants with a confirmed diagnosis of achondroplasia and AFMS3 on initial MRI who subsequently underwent repeat MRI or proceeded straight to neurosurgery., Results: Twenty-two cases satisfied the inclusion criteria. Mean age in months was 6.23 (SD±3.82) and 17.95 (SD±7.68) at baseline and follow-up scans, respectively. Follow-up MRI showed no change in 23% (N=5), improvement in 36% (N=8) to either AFMS1 (N=5) or AFMS2 (N=3). There was progression in 41% to AFMS4 (N=8). One case had neurosurgey without follow-up MRI (N=1)., Conclusions: These results support MRI screening for FMS in infants with achondroplasia. Furthermore, infants with AFMS3 should undergo follow-up MRI as over 40% progress prompting neurosurgical intervention. There is currently no consensus on frequency or timing of screening for AFMS3 in achondroplasia; however, we suggest that guidance for follow-up imaging is modified to 6 months to detect progression earlier in this at-risk cohort., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2024. No commercial re-use. See rights and permissions. Published by BMJ.)