A new prenatal blood test that’s becoming increasingly popular is extremely accurate in detecting Down syndrome very early in a woman’s pregnancy, according to a study led by scientists at the University of California, San Francisco, but whether it should replace the current standard testing is still not clear.

The study, one of the largest to compare two types of prenatal screening tests, does not give a definitive answer on which test women and their obstetricians should choose. But it offers the strongest evidence so far that the newer test, which analyzes fetal DNA in the mother’s blood, is far superior to standard testing for Down syndrome.

“If one is thinking about screening for Down syndrome — about screening options for that one condition — it is very clear that DNA testing is the best for that,” said Mary Norton, a UCSF obstetrician and lead author of the study, which was published this week in the New England Journal of Medicine.

“But this test, while very exciting, tests for a very narrow group of disorders,” Norton said. “For a given woman thinking about her test options, what should be our first-line test is still not so clear.”

Obstetricians use blood tests to screen pregnant women for dozens of fetal genetic disorders, many of them caused by chromosomal abnormalities. Down syndrome accounts for roughly half of all chromosomal disorders and up to 10 percent of all other birth defects associated with intellectual disabilities.

The standard screening test looks for protein markers in the mother’s blood that suggest a chromosome disorder. Standard screening also includes an ultrasound that looks for extra fluid in the fetus’ neck, another sign of chromosomal abnormalities.

The newer screening procedure, called cell-free DNA analysis and first made available to consumers in 2011, looks at scraps of fetal genetic material found in the mother’s blood.

For all screening tests, positive results must be confirmed by amniocentesis or chorionic villus sampling, both of which involve collecting a sample of the fetus’ placenta or the amniotic fluid to perform rigorous lab tests. Because amniocentesis and chorionic villus sampling both carry a small risk of miscarriage — about 1 in 350 cases — the tests generally aren’t done unless a woman is in a high-risk category, usually based on her age or on a screening that suggests the fetus may have a birth defect.

In the UCSF research, the DNA test identified all 38 fetuses with Down syndrome from nearly 16,000 women studied; there were only nine false positive results. That’s far more accurate than the standard screening test, which found 30 of 38 Down syndrome cases in the study and had 854 false positives.

The results weren’t entirely surprising, but “I don’t know that we’ve had it laid out as clearly as we have it laid out here,” said Arzou Ahsan, an obstetrician with East Bay Physician Medical Group who practices out of the Alta Bates Summit Medical Center in Berkeley, Calif.

“(The DNA test) is superior in regards to its ability to predict the presence of a chromosomal defect,” Ahsan said. “As we expand its usage, I think it will help us do a lot fewer amniocenteses, which are very stressful for patients.”

But the DNA test was designed to screen for Down syndrome and a handful of other chromosomal disorders, and it’s not useful in testing for most genetic abnormalities.

In a sense, women and their physicians have to decide whether they prefer very reliable screening for the single most common chromosomal defect, or a less reliable test for a much wider array of disorders.

For women age 35 and over — who face the greatest risk of having a child with Down syndrome — the newer test may be the best choice, Norton said. The risk of having a child with Down syndrome is about 1 in 1,000 for women under age 35, but it climbs to 1 in 400 at age 35 and 1 in 100 at age 40.

The standard blood-screening test may be a better option for younger women, Norton said. She noted that doctors in some European countries recommend that pregnant women get the standard screening test first and the DNA test next before moving on to more invasive techniques. That’s a similar solution to many that U.S. physicians have worked out with their patients.

“It’s sort of a multi-stepped process, and I think that’s perfectly fine for now,” said Jane Chueh, director of prenatal diagnosis and therapy at Stanford University.

Choosing multiple screening tests, however, is not a solution for many women based on cost alone. The standard screening tests cost about $200 and are almost always covered by insurance. The DNA testing sometimes will be covered by insurance, especially if a woman is 35 or older, but many patients have to pay for it themselves at a cost of up to $2,000.

Chueh said she typically only brings up DNA screening with older patients, then discusses the risks and benefits of each test before deciding what to do. Increasingly, however, she’s seeing patients who have done their own research and ask for the newer DNA screening, even if they’re not considered a high-risk pregnancy.

“A lot of people are doing everything, all the testing, especially if they have the resources,” Chueh said. “It’s hard to decide. You want everything, you want to make sure everything turns out OK. You don’t want to give up that safety.”

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