Background and objectives: Sex chromosome trisomy (SCT) is a common chromosomal abnormality associated with increased risks for early developmental delays and neurodevelopmental disorders later in childhood. Our objective was to quantify the spectrum of early developmental milestones in SCT. We hypothesized later milestone achievement in SCT than the general population. Methods: Data were collected as part of the eXtraordinarY Babies Study, a prospective natural history of developmental and health trajectories in a prenatally identified sample of infants with SCT. Parent reported, clinician-validated, early motor and language milestones were collected at 2, 6, 12, 18, 24, and 36-months. Age distributions of milestone achievement were compared with normative data. Results: In all SCT conditions, compared with normative data, there was increased variability and a later median age of skill development across multiple gross motor and expressive language milestones. Results also show a significant amount of overlap with the general pediatric population, suggesting that for many children with prenatally identified SCT, early milestones present within, or close to, the expected timeline. Conclusions: As increasing numbers of infants with prenatal SCT diagnoses present at pediatric practices, we provide an evidence-based schedule of milestone achievement in SCT as a tool for pediatricians and families. Detailed data on SCT milestones can support clinical interpretation of milestone achievement. Increased variability and later median age of milestone acquisition in SCT compared to norms support consideration of all infants with SCT as high risk.

The authors have declared no competing interest.

This study was funded by the eXtraordinarY Babies Study: Natural History of Health and Neurodevelopment in Infants and Young Children with Sex Chromosome Trisomy (NIH NICHD R01HD091251, 3R01HD091251-05S1)

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ClinicalTrials.gov NCT03396562; Colorado Mutliple Institutional Review Board (COMIRB) 17-0118; Nemours Institutional Review Board# 1151006. Consents were signed by parents or legal guardians of all participants under age 18.

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