A Healthy, Female Chimera with 46,XX/46,XY Karyotype

We report a healthy and unambiguously female newborn, whose phenotypic sex contradicted the expected male sex based on previously performed prenatal cytogenetic analysis. Both 46,XX and 46,XY cells were detected in a villus sample, the former having been attributed to maternal cell contamination. Postnatal karyotyping in peripheral lymphocytes confirmed the presence of two cell lines, one 46,XX (70%) and one 46,XY (30%). After exclusion of alternative explanations for the observed genotype, a diagnosis of chimerism was made. Chimeras containing cell lines of opposite sex usually feature ovotesticular development with associated genital ambiguity. To account for the normal female appearance of our patient, we postulate the exclusive involvement of 46,XX cells in gonad formation.