Prenatal interphase detection by FISH of a sex chromosome mosaicism when cytogenetics reports a pseudomosaicism

We report the detection of a sex chromosome mosaicism (XY/XXY/XYY) by prenatal interphase FISH (fluorescence in situ hybridization) originally identified as a pseudomosaicism involving 47,XXY cells present in a routine 46,XY cytogenetic analysis. After a fetal demise, interphase FISH identified the sex chromosome mosaicism in all tissues examined while cytogenetic analysis revealed only a normal male cell line. After prolonged exposure to colcemid, cytogenetic analysis identified the 47,XXY cell line. This confirmed the presence of the mosaicism and suggested that the abnormal cell line(s) may have been growth disadvantaged. This in turn may have accounted for the intra-uterine fetal demise. The identification by FISH and the role of growth-disadvantaged cell lines may provide a unique insight into chromosomally normal fetal demises.