A 1.57-kg infant presented at a major medical center in the southeastern United States at 32 weeks of gestation with growth restriction and no major anomalies after an uncomplicated pregnancy. At 1 month of life, the infant was found to be chimeric for blood types O and A. Genetic testing revealed mosaic trisomy 9 as the cause for the 2 distinct blood types. Without phenotypic presentation of trisomy 9, the infant's genetic diagnosis was not detected until an issue arose. Genetic diagnosis and treatment and future considerations are discussed in this article. Full-text English articles from CINAHL and PubMed were analyzed for assistance in understanding the infant's condition. Book chapters, review articles, and meta-analyses were also reviewed. Implications of this case study indicate that phenotypically normal presenting infants may still have underlying issues that should be investigated genetically when they arise. This article cannot be generalized to the population because of its specific situation, but the underlying concept can be applied to any case.