PROJECT ABSTRACT Polyunsaturated fatty acids (PUFA) are critical for normal metabolism and neurodevelopment, and the precursors for PUFA synthesis need to be obtained through diet or dietary supplementation. Individuals with an additional X or Y chromosome, referred to as sex chromosome trisomy (SCT) conditions, are susceptible to both impaired metabolism and neurodevelopment, although mechanisms are unknown and no treatments are available. Recently, we found lower plasma PUFA concentrations in an SCA cohort compared to controls. Diet, supplement use, and PUFA intake specifically has not been assessed in SCA. The eXtraordinarY Babies Study (R01HD091251) is a longitudinal natural history of health and neurodevelopment in >200 prenatally identified infants with SCT with the overarching goal to identify risk and protective factors contributing to the spectrum of variability in neurodevelopmental outcomes in these individuals. In this supplement to the eXtraordinarY Babies Study, we will rigorously quantify intake of PUFA and other FA through diet and supplements through standardized parental interviews (Aim 1). We will then assess the relationship between exogenous intake of PUFA with plasma concentrations using targeted metabolomics analysis of stored samples at two time points (Aim 2). Finally, we explore the relationship of PUFA intake to direct neurodevelopmental assessments being collected at study visits that measure cognitive, language, motor, and social development (Aim 3). Results of these supplemental aims will be added to the development of our comprehensive model aimed at understanding the genetic and environmental predictors of phenotypic variability among children prenatally identified to have SCA.