DNA blood tests could improve prenatal screening

A DNA test of a pregnant woman’s blood is more accurate than current methods of screening for Down syndrome and other common disorders, new research finds. If other studies bear this out, it could transform prenatal care by giving a more reliable, noninvasive way to detect these problems very early in pregnancy.

That would let couples decide sooner whether to have an abortion or to prepare for a major medical problem. It also might cut down on the 200,000 more-invasive tests like amniocentesis done each year in the United States to diagnose or rule out problems with a fetus.

“It offers women a safe and accurate alternative” for screening, said the study leader, Dr. Diana Bianchi, of Tufts Medical Center in Boston.

Several companies already sell these DNA blood tests, which can be done when the fetus is only 9 to 10 weeks old, a of couple weeks sooner than current methods. They screen for disorders caused by extra or missing chromosomes, such as Down syndrome, which occurs in about one of every 700 pregnancies.

Current screening methods are imprecise. Ultrasounds and various blood tests can hint at a problem but don’t directly test for one. The next step is diagnostic testing – amniocentesis, like a needle biopsy to collect fetal cells, or chorionic villus sampling, which takes a snip of the placenta. Both bring a small risk of miscarriage.

The DNA tests aim to improve screening and lower the number of women referred for these more invasive tests. Using a sample of the mother’s blood, they sequence the alphabet of bits of DNA shed from the placenta and map them to various chromosomes. The numbers are compared to what’s normal at that stage of pregnancy.

Groups like the American College of Obstetricians and Gynecologists say these DNA tests can be an option for higher risk pregnancies – mothers 35 and older, those with an abnormal ultrasound or blood test, or with a prior pregnancy involving abnormal chromosomes – but their accuracy in the general population isn’t known.

The new study, published Wednesday by the New England Journal of Medicine, is the first to look at their use in a general U.S. population. Nearly 2,000 women had standard prenatal screening plus a fetal DNA test from Illumina Inc., a California company that sponsored the study.

Both methods detected eight chromosome abnormalities, including five of Down syndrome.

But the false alarm rate for Down syndrome with DNA testing was just a fraction of that for standard screening – 0.3 percent versus 3.6 percent. DNA tests also gave fewer false alarms for Edwards and Patau syndromes, less common chromosome abnormalities.

That means far fewer women would be advised to get diagnostic tests to rule out a problem.

Several independent experts called the new research a good first step, but not enough to warrant using the DNA tests now in the general population.

“It’s encouraging,” but doctors will want to wait for other and larger studies underway now of various fetal DNA tests, said Dr. Nancy Rose, a University of Utah professor who heads the genetics committee of the College of Obstetricians.

Dr. Susan Klugman, director of reproductive genetics at New York’s Montefiore Medical Center, noted that nearly one-third of the women in this study had the DNA testing in their third trimester of pregnancy, and said doctors will want to see more evidence of how well DNA testing does in the earliest months.

“Cost is a huge factor,” Klugman said.

Four companies sell the tests for $1,200 to $2,700, said Bianchi, the study leader, who has consulted for Illumina. Blood tests used for screening now cost $300 to $400, and ultrasound costs $200 to $300. Insurance coverage varies, depending on whether women are at higher risk for having a fetus with chromosome abnormalities, she said.