Down syndrome occurs in approximately one in 1,000 live births worldwide. Children with Down syndrome have intellectual and some physical disabilities. Many pregnant women get tested for the genetic disorder. The traditional prenatal test for Down syndrome has been around for decades but comes with risks to the fetus. Now, researchers have developed a non-invasive blood test that’s very accurate in detecting the congenital disorder.
Babies with Down syndrome, also known as trisomy have an extra copy of chromosome 21, which causes what used to be called mental retardation. They also have a number of characteristic physical features, including short stature, a broad face, slanted eyes and a short neck.
Traditional testing for fetal abnormalities, called amniocentesis, involves inserting a long, thin needle into the womb to extract amniotic fluid from the placenta, or the fetal sac, and testing it for certain protein markers.
The test is most often performed on women ages 35 and older who are considered at highest risk for giving birth to a baby with Down syndrome. The test can also identify other congenital diseases, including sickle cell anemia and Tay-Sachs disease.
Amniocentesis, performed between 14 and 16 weeks of pregnancy, carries a small risk of miscarriage. CVS, or chorionic villus sampling, takes a small bit of tissue from the placenta and tests it for Down syndrome and other congenital abnormalities. It too carries a small risk of miscarriage.
A small percentage of DNA circulating in the mother’s bloodstream is from the fetus, and a new test, developed at the University of California, San Francisco, takes advantage of that.
Mary Norton, an expert in high-risk obstetrics and clinical genetics at the school, says the non-invasive blood test analyzes this genetic material.
“With advances in genetics, one can sequence that DNA, and measure the amount that comes from each of the chromosomes," said Norton. "And by doing that, you can say, ‘Oh, look there’s extra here that comes from chromosome 21 that’s consistent with the fetus having Down syndrome.’”
Called the cell-free DNA blood test, it can be performed earlier in a woman’s pregnancy than the other procedures, between 10 and 14 weeks.
“There are some false positives and false negatives, so we do still consider it a screening test that needs to be confirmed by a CVS or amniocentesis if the results are abnormal. But it’s far, far more accurate than the last generation screening tests that are currently being done most of the time now," said Norton.
In a study of 16,000 pregnant women published in The New England Journal of Medicine, Norton and colleagues describe analyzing fetal DNA circulating in the women’s bloodstream.
Thirty-eight fetuses were identified with Down syndrome compared to 30 using standard methods of screening.
The cell-free blood test also yielded a smaller number of false positives than conventional pregnancy screening; however, it does not identify all the abnormalities that the other procedures do, and Norton encouraged doctors to stress that limitation in counseling women who choose to take the test.