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A new study reported this week shows UK researchers have developed a more reliable non-invasive test for Down syndrome during the first three months of pregnancy. Writing in the 7 June online issue of Ultrasound in Obstetrics & Gynecology, Kypros Nicolaides of King's College London, and colleagues, suggest the test, which analyzes the baby's DNA in the expectant mother's blood, is superior to currently available non-invasive ways of screening for Down syndrome. In the UK every year, around 750 babies are born with Down syndrome. The condition, which is also known as trisomy 21, is one of the most common genetic causes of learning disability. Down syndrome is caused by the presence of an extra copy of chromosome 21 in the baby's cells and occurs by chance at conception. The risk of having a baby with the condition increases with maternal age. At present, to find out if her baby has Down syndrome, a pregnant woman can have a "combined test" between the 11th and 13th week of her pregnancy. This test, which involves an ultrasound scan and a blood test and takes the woman's age into account, gives her an estimate of the chance of her baby having Down syndrome. From those results she can then decide whether to have a more invasive test that will tell her definitely whether her baby has the condition. But an invasive test can increase the risk of miscarriage. There are two invasive tests for Down syndrome, chorionic villus sampling (CVS), which samples a small piece of the placenta, and amniocentesis, which tests the amniotic fluid around the baby. Several studies have already suggested that non-invasive genetic tests that use fetal cell free DNA (cfDNA) from a pregnant woman's blood can be highly sensitive and specific. This latest study from Nicolaides and colleagues is the first to look at the feasibility of using such a cfDNA test for trisomy 21 and other chromosomal disorders. Their results, obtained from tests carried out in 1,005 pregnancies at 10 weeks, showed a lower rate of "false positives" and higher sensitivity than the combined test done at 12 weeks. The rate of false positives using the cfDNA test was 0.1% compared to 3.4% for the combined test. The authors conclude: "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 favor of or against invasive testing," they add. A second study by the same team, reported in the same issue of Ultrasound in Obstetrics & Gynecology, implies it might be worth combining the cfDNA test done at 10 weeks with the combined test at 12 weeks. The study concludes that "screening for trisomy 21 by cfDNA testing contingent on the results of an expanded combined test" would not only retain the advantage of the current screening method, but also significantly increase rates of detection and decrease rates of invasive testing. For that study the team examined results from tests done on pregnancies at three UK hospitals between 2006 and 2012. Earlier this year in the same journal, Nicolaides and colleagues reported a study where they found certain ultrasound markers for Down syndrome are more significant than others. |
