Trisomy 21/mosaic Turner detected in fetus by non-invasive prenatal testing

Non-invasive prenatal testing (NIPT) is a screening method for the early detection of foetal aneuploidies in
pregnant women. While originally developed for the detection of trisomy 13, 18 and 21, it is becoming clear
that NIPT can be used for the identification of rare foetal aneuploidies and mosaic aneuploidy as well. Here we
describe a female foetus with trisomy 21 in combination with mosaicism X0/XX, detected during follow-up of
an abnormal ultrasound (enlarged NT: 3,3 mm). Microarray analysis and FISH on chorion villi cells confirmed
trisomy 21, and mosaicism X0/XX (~29% of cells (n=63)). NIPT convincingly detected the presence of trisomy 21
with a Z-score of 22.9. (Partial) monosomy X on NIPT was revealed by the absence of a clear second X
chromosome on the sex plots and the high discrepancy between the foetal fraction of chromosome X and the
foetal fraction of chromosome Y. Likely the high seqFF value (16%) helped to more confidently identify these
aneuploidies. While the debate as to whether or not aneuploidies of the sex chromosomes should be reported
continues, this study shows that these aneuploidies can nevertheless be picked up, and probably other rare
genetic abnormalities as well.