551 CHROMOSOMAL MOSAICISM IN AMNIOTIC FLUID CELL CULTURES

The ability to differentiate true from pseudomosaicism is of major concern in prenatal diagnosis. Of 1000 amniotic fluids processed by in situ methods, a total of 26 demonstrated some degree of chromosomal mosaicism. Two, which had more than one colony possessing the same abnormality, were interpreted as true mosaicism. In both cases the same mosaicism (45,X/46,XX and 46,XX/47,XX+21) was shown to be present in the newborn or fetus. The remaining 24 cases, including 10 with trisomy 2, were interpreted as pseudomosaicism since only a single colony or partial colony demonstrated the aberrant chromosome complement. No phenotypic abnormalities have been noted in the babies delivered following a diagnosis of pseudomosaicism.