Natural History of Prenatally Identified Children with 48,XXYY Syndrome in Infancy and Early Childhood.

Background: 48,XXYY syndrome is a rare sex chromosome aneuploidy (SCA) characterized by neurodevelopmental deficits and medical comorbidities. The limited information available in the literature is almost exclusively limited to postnatally diagnosed cases. This study aims to describe the early medical and developmental features of prenatally identified 48,XXYY infants, with comparisons to 47,XYY, 47,XXY cohorts, and typical populations, as well as previously reported postnatally diagnosed 48,XXYY cases. Methods: The eXtraordinarY Babies Study prospectively follows children prenatally identified to be at high risk for SCA with annual medical and neurodevelopmental evaluations. Data presented herein include the prevalence of medical conditions, developmental milestones, developmental and adaptive functioning assessment scores, and therapy utilization in participants confirmed to have 48,XXYY. Comparisons were made between this cohort and the typical population, infants with 47,XYY and 47,XXY also enrolled in the eXtraordinarY Babies Study, and a 2008 cohort of individuals postnatally identified 48,XXYY. Results: Infants with 48,XXYY exhibited a range of early medical features, including high rates of feeding and GI disorders (breastfeeding difficulties, gastroesophageal reflux, and eosinophilic esophagitis), allergic disorders (food allergies and environmental allergies), and hypotonia. Developmental and adaptive functioning scores indicated delays in motor, communication, and social domains, with nearly all infants receiving speech therapy, physical and/or occupational therapy. Comparisons with the 47,XYY and 47,XXY cohorts revealed more medical and developmental challenges in the 48,XXYY group, however there was variability and some overlap with both the general population and sex chromosome trisomy conditions. Additionally, comparison to the 2008 postnatally identified 48,XXYY cohort indicated that while prenatal diagnosis allowed for earlier intervention, developmental outcomes in the first years of life were similar between the two groups. Conclusions: 48,XXYY diagnosed prenatally facilitates early monitoring, anticipatory guidance, and proactive referrals for medical evaluations and intervention, given developmental delays and medical challenges are more common in infancy and early childhood compared to the general population and trisomy SCAs. These findings provide valuable insights for genetic counselors and healthcare providers, emphasizing the spectrum of medical and developmental findings and importance of early and proactive care to support individual outcomes. Prospective study of this prenatally identified cohort will provide important natural history and phenotypic variability in XXYY, as well as identification of predictors of health and developmental outcomes.