Chromosomal mosaicism in prenatal diagnosis: a problem still unsolved

The problems in differentiating chromosomal mosaicism from pseudomosaicism after amniocentesis and CVS are demonstrated in 6 cases. Two cases of true mosaicism (45, X/46, XX and 46, XY/47, XXY) were of clinical relevance. In both cases the aberrant cell line was less expressed in amniotic fluid cells than in fetal blood and cultivated fibroblasts. Two cases of pseudomosaicism (chromosome 2 and chromosome 10 trisomy) originated either from preexisting mutants or from in vitro mutations whereas a case of true chromosome 20 mosaicism indicated the possibility of a mosaic confined to a single fetal tissue. The problems of interpreting mosaicism after CVS is illustrated in a 45, X/46, XY case, in which the abnormal cell line was detectable only in extrembryonic tissue.