Fetal aneuploidy affects about 9 in 1,000 live births. The definition of aneuploidy is an abnormal number of chromosomes ; with 23 pairs of chromosomes in humans, 46 is the normal number, while aneuploidy individuals will have 45 or 47. In trisomy, there is one additional chromosome, typically chr21, 18 or 13 (it is not a coincidence that these are the smallest chromosomes in humans). Historically, the invasive methods amniocentesis and chorionic villus sampling (CVS) were used with risk to the pregnancy, with about a 1% chance of miscarriage due to the procedure. Non-invasive methods based upon ultrasound and serum biomarkers are useful screening tests, but were of limited reliability as they were indirect measures of chromosomal abnormalities1.Photograph courtesy of Flickr user Can H.