Possible mechanism for the origin of double aneuploidy mosaicism in this fetus. Schematic representation of the mechanisms that may have lead to a mosaic pattern of both trisomy 7 and trisomy 13 cells in a single fetus. Two independent non-disjunction events may have taken place in a 46, XX zygote, resulting in both a trisomy 7 cell line (26%) and a trisomy 13 cell line (70%). Both monosomic cells are not viable. In fetal tissue, 4% of the cells in fetal tissue showed a normal signal pattern with FISH.