12/13/2011 12:55 pm ET | Updated Feb 12, 2012

Does My Baby Have Down Syndrome? Say 'Yes' or 'No'

The new maternal blood test to diagnose fetal Down syndrome in pregnancy is getting a lot of buzz. The first test went on the market last month with reasonably good predictive statistics: 99 percent accurate. It's not perfect, but it's pretty darn close -- much closer than any other non-invasive test (if the company currently offering the test is to be believed).

It's not for everyone -- only women at high-risk for delivering a baby with Down syndrome are recommended to consider the test -- but it's the first really new option in prenatal diagnosis in many years.

My colleagues at Tufts Medical Center's Mother Infant Research Institute and Center for Perinatal Diagnosis recently teamed up with Carey Goldberg from WBUR's Commonhealth Blog to explain the new test to patients. Find their really clear answers to common questions here.

In a prior post I've poo-pooed the test and suggested that it will take years to gain acceptance. But if test provider, Sequenom, or another one of the companies working on similar tests, can make that 99 percent accurate number turn into a 100 (or even a 99.8), maybe they have a chance. The simple reason is that, like amniocentesis or chorionic villus sampling, the test speaks clearly: "yes" or "no." Other tests are maddeningly equivocal.

Warning: befuddling statistics ahead.

The best non-invasive "screening" test for Down syndrome that is currently standard of care identifies about 95 percent of affected fetuses. Seems straightforward? It's not.

Let's follow Jane, a hypothetical patient, through the process.

The "sequential" test combines tests done in the first and second trimester. It begins with Jane's risk of having a child with Down syndrome based on her age alone. Jane will be 35 at delivery; her risk is approximately one in 240. This is because as women age, their egg cells age, and there is a higher risk of chromosome problems.

Now Jane goes for an ultrasound exam to measure the "nuchal translucency," a space at the back of the fetus' neck. I can't tell you the number of times women who have asked if their child will have Down syndrome after I'm done with the ultrasound, but as a stand-alone test, the ultrasound usually tells very little.

Then Jane has her blood drawn for the first time and goes home.

A few days later, the first results came to Jane's doctor. The results were, "screen negative." Patients often see this as a "yes" or "no" result, but it isn't. Screen positive only means that the risk is more than one percent (or more than two percent in some labs). Screen negative doesn't mean "no Down syndrome," it only means that Jane's risk is less than 1 percent (or 2 percent).

Now Jane waits, and 5-6 weeks later she has more blood drawn. Days later, the results come back "screen positive," with a risk of 1:200. What does this mean? I explain this risk to Jane in three ways:

First, I say, "your risk is 1 in 200" (blank stare).

Then, I say, "What that means is that if there are 200 women in a room who are exactly the same age as you and have the same results, one of them is carrying a child affected with Down syndrome, and the other 199 are carrying normal children; it's just that we don't know whether you are the 199 or the one" (a glimmer of understanding).

Then I say, "Another way to think about it is that your chance of having a baby affected with Down syndrome is about one-half of one percent" (the blank stare returns).

To complicate matters, I now add, "If you want to know for certain, you can have an amniocentesis, which will give you a yes or no answer, but that test creates a small risk of miscarriage."

To be fair, very often the test reports a "screen negative" result with a 1 to 10,000 risk of Down syndrome, which is to say that the chance that the patient carries a Down syndrome-affected fetus is vanishingly small, and I say "extremely unlikely," and patients are often reassured.

Now, remember that 95 percent accurate figure from the beginning of this page-long description of the sequential screening test? What that means is that 95 percent of the time fetuses with Down syndrome cause the test to fall into the "screen positive" category (but 5 percent of the time their moms have "screen negative" results).

My patients come to me, typically, after having spent 45 minutes with a genetic counselor and I find that many (if not most) are still confused about these tests. In medicine, like in most of life, we want certainty. For 25 years we have counseled our patients about risks and screening tests and invasive tests that carry the risk of miscarriage, and our patients are anxious and confused and frustrated.

If we could offer them a blood test that provided certainty without the risk of losing the pregnancy, it would be a good thing.