A new noninvasive and highly accurate screening test that analyzes a sample of fetal DNA fragments found in expectant mothers blood, is reducing the number of women who undergo amniocentesis an invasive procedure used in prenatal diagnosis of chromosomal abnormalities.
Amniocentesis is a procedure in which a thin needle is introduced into the pregnant woman s fetal sac to withdraw fluid and sample amniotic fluid for substances and cells, and subject them to genetic typing. It is usually done after the first trimester. Prenatal screening has gained ground in the last four decades, after amniocentesis was widely accepted in the 70s. Today, there are more than 800 prenatal tests for various genetic disorders, including this new screening test which detects Down Syndrome, or trisomy 21 (three copies of chromosome 21 when there should be only two). It is the most common chromosomal abnormality, occurring in one of every 700 U.S. births.
Unlike amniocentesis which can sometimes cause preterm labor and fetal trauma if conducted earlier than the 15th week of pregnancy the new noninvasive test relies on a few drops of the mother s blood, which contains free DNA from both herself and her unborn child. According to Dr. Diana W. Bianchi, a neonatologist and geneticist at Tufts University School of Medicine, the test reduces cost, complications and provides exceptional accuracy. Although not yet approved by the Food and Drug Administration, Dr. Bianchi says any woman can request it, provided she is willing to cover the costs.
Older tests like amniocentesis and chorionic villus sampling (C.V.S.) detect roughly 92 percent of Down Syndrome cases, with false-positives incorrectly showing Down Syndrome in about five percent of cases. With the new prenatal screening, fewer than 1 percent of tests are falsely positive.
ACSH s Dr. Gilbert Ross said, This is clearly a major advantage for women who, for whatever reason, want to know the likelihood that their baby will have Down syndrome or some other genetic variant. The high predictive power and low risk as compared to amniocentesis is a significant benefit, assuming these data are reproducible.