Blood test for pregnant women diagnoses Down's Syndrome in foetus
7 June 2013
A non-invasive test that analyzes foetal DNA in a pregnant
woman’s blood can accurately detect Down’s syndrome and other
genetic foetal abnormalities in the first trimester.
The results suggest that the test is superior to currently
available screening strategies and could reshape standards in
prenatal testing. The research has been published in two papers in
the journal Ultrasound in Obstetrics & Gynecology. [1,2]
Current screening for Down's syndrome, or trisomy 21, and other
trisomy conditions includes a combined test done between the 11th
and 13th weeks of pregnancy, which involves an ultrasound screen and
a hormonal analysis of the pregnant woman’s blood. Only chorionic
villus sampling and amniocentesis can definitely detect or rule out
foetal genetic abnormalities, but these are invasive to the
pregnancy and carry a risk of miscarriage.
Several studies have shown that non-invasive prenatal diagnosis
for trisomy syndromes using foetal cell free (cf) DNA from a
pregnant woman’s blood is highly sensitive and specific, making it a
potentially reliable alternative that can be done earlier in
A study by Kypros Nicolaides, MD, of the Harris Birthright
Research Centre for Fetal Medicine at King’s College London in
England, and colleagues is the first to prospectively demonstrate
the feasibility of routine screening for trisomies 21, 18, and 13 by
cfDNA testing. Testing done in 1005 pregnancies at 10 weeks had a
lower false positive rate and higher sensitivity for fetal trisomy
than the combined test done at 12 weeks. Both cfDNA and combined
testing detected all trisomies, but the estimated false-positive
rates were 0.1% and 3.4%, respectively .
“This study has shown that the main advantage of cfDNA testing,
compared with the combined test, is the substantial reduction in
false positive rate. Another major advantage of cfDNA testing is the
reporting of results as very high or very low risk, which makes it
easier for parents to decide in favour of or against invasive
testing,” the authors wrote.
A second study by the group, which included pregnancies
undergoing screening at three UK hospitals between March 2006 and
May 2012, found that effective first-trimester screening for Down’s
syndrome could be achieved by cfDNA testing contingent on the
results of the combined test done at 11 to 13 weeks. The strategy
detected 98% of cases, and invasive testing was needed for
confirmation in less than 0.5% of cases. 
“Screening for trisomy 21 by cfDNA testing contingent on the
results of an expanded combined test would retain the advantages of
the current method of screening, but with a simultaneous major
increase in detection rate and decrease in the rate of invasive
testing,” the authors concluded.
1. MM Gil, MS Quezada, B Bregant, M Ferraro, and KH
Nicolaides. Implementation of maternal blood cell-free DNA testing
in early screening for aneuploidies. Ultrasound in Obstetrics &
Gynecology; Published Online: June 7, 2013 (DOI: 10.1002/uog.12504).
2. KH Nicolaides, D Wright, LC Poon, A Syngelaki, and M Gil.
First-trimester contingent screening for trisomy 21 by biomarkers
and maternal blood cell-free DNA testing.” . Ultrasound in
Obstetrics & Gynecology; Published Online: June 7, 2013 (DOI: