Detecting Down Syndrome in early pregnancy
A blood test undertaken between 10 to 14 weeks of pregnancy may be more effective in diagnosing Down syndrome and two other less common chromosomal abnormalities than standard non-invasive screening techniques, according to a multicentre study led by a UC San Francisco researcher.
In the study, which followed pregnancy outcomes in close to 16,000 women, the cell-free DNA blood test resulted in correctly identifying all 38 foetuses with Down syndrome, a condition associated with cognitive impairments and an increased risk of several medical disorders. The diagnosis was confirmed by newborn exam, prenatal or postnatal genetic analysis.
The test focuses on the small percentage of foetal DNA found floating in a pregnant woman’s blood. DNA is amplified by PCR, and sequenced so that comparisons can be made between relative amounts of each chromosome’s DNA. A greater quantity of DNA is indicative of some chromosomal conditions, including Down syndrome, which is characterized by an extra copy of chromosome 21, one of the 23 pairs of chromosomes.
When the same women underwent standard screening, 30 of the 38 foetuses with Down syndrome were flagged, according to the study published on April 1, 2015, in the New England Journal of Medicine. The screening comprises a blood draw in which hormones and proteins associated with chromosomal defects are identified, together with an ultrasound of the nuchal fold fluid in the back of the neck, an excess of which is suggestive of Down syndrome.
The average age of the pregnant women was 30 and approximately one-quarter were over 35 – the age at which women have traditionally been considered high risk and offered prenatal invasive testing with procedures like amniocentesis.
A second compelling advantage of cell-free DNA analysis, reported by the researchers who were led by first author Mary Norton, MD, professor of clinical obstetrics and gynaecology at UCSF, was the relatively low incidence of Down syndrome misdiagnoses. While standard testing is acknowledged to result in a large number of false positives, these were significantly less likely with the cell-free DNA tool. There were nine false positives resulting from this method, vs. 854 with standard screening. University of Central Florida
