Tissue specific trisomy--the significance of confined placental mosaicism.

Confined placental mosaicism is tissue specific chromosomal mosaicism arising in both chromosomally normal and abnormal zygotes. Studied by first trimester chorionic villus sampling it has been reported in about 1-2% of human gestations. It is more common in the cytotrophoblast than in placental connective tissue. While intrauterine survival of some chromosomally abnormal embryos and fetuses appears to be significantly enhanced by its presence, the effect of confined placental mosaicism on chromosomally normal fetuses is variable and at present time still unpredictable.