Recovering and analysing fetal erythrocytes from maternal blood is bei
ng pursued for non-invasive prenatal genetic diagnosis. We report the
observation of 46,XY/47,XXY mosaicism in fetal cells from a woman whos
e first-trimester chorionic villus sampling (CVS) initially showed onl
y 46,XY. Only after exhaustive (500 cells) analysis were four XXY cell
s found in cultured villi.