To amnio or not to amnio? New test can provide clearer answer for moms-to-be

The thought of a 3-inch needle made me cringe. But the risk of miscarriage was what really worried me.

So, after talking with my husband, my doctor and a genetic counselor, I decided to forgo amniocentesis, a test that could have told me whether my unborn baby had Down's syndrome. My doctor had recommended the test based on my "advanced maternal age" of 36, and results of a blood test suggesting a higher-than-average risk of Down's syndrome. Although I never second-guessed my decision not to have the amnio, it did leave me with some lingering anxiety about my baby, which thankfully was dispelled when my son was born healthy.

Whether or not to have amniocentesis—or a related test called chorionic villus sampling—is a difficult decision for many expectant parents. These tests can tell you for certain whether your unborn baby has Down's syndrome, by checking the baby's DNA for this genetic disorder. But the tests aren't without risk, as they involve putting a needle into the womb to either withdraw fluid (amniocentesis) or remove a small piece of the placenta (chorionic villus sampling). A small number of women have a miscarriage as a result—around 1 in 100, by some estimates.

But now a new study suggests these tests—and the worries they inspire—may soon be a thing of the past for most pregnant women. Researchers have found that a new screening test, done on a blood sample from the mother, can completely rule out Down's syndrome if her baby does not have the condition. The test, called multiplexed massively parallel DNA sequencing, was 100 percent accurate—it never showed a baby didn't have Down's syndrome when it actually did.

It was also very accurate at confirming cases of Down's syndrome, but not 100 percent. Of the women whose tests were positive for the syndrome, only 98 in 100 actually had a baby with the condition. So, for 2 in 100 cases, the test said the baby had Down's syndrome when it didn't.

The researchers say their results mean that only women who had a positive result would need to consider having one of the more invasive tests, to confirm Down's syndrome. This would apply to only 1 in every 1,000 pregnant women—a fraction of the number offered these tests today.

The study looked only at women who were at high risk of having a baby with Down's syndrome. So we don't know how accurate it would be for women not in a high-risk group. It's possible more women in a low-risk group would be given false-positive results.

Even so, these findings provide good news for many future moms-to-be, who may no longer need to anxiously weigh the benefits and risks of the more invasive tests.

What you need to know. A new blood test can accurately rule out Down's syndrome among pregnant women at high risk, without the need for more invasive testing. A commercial version of the test is reportedly now under development. If you are currently pregnant and have questions about screening tests, or about Down's syndrome, speak to your doctor. He or she can tell you more about the options available.

—Sophie Ramsey, patient editor, BMJ Group

 ConsumerReportsHealth.org has partnered with The BMJ Group to monitor the latest medical research and assess the evidence to help you decide which news you should use.

—Aaron Bailey