A Promising New Way To Screen For Down’s Syndrome
By Lissa Rankin on January 13, 2011
BlogHer Original Post
Hey Moms, do you remember the heart-wrenching decisions you had to make about prenatal diagnostic testing when you were pregnant? Should you do the triple-screen AFP test at 15 weeks? Or should you do the nuchal translucency ultrasound plus blood tests for hormone markers at 11-13 weeks. Or should you suck it up and go for the gold standard -- chorionic villus sampling (CVS) or amniocentesis?
When I was pregnant with my now 5-year old daughter, I wanted to know. As I wrote in my book What’s Up Down There? Questions You’d Only Ask Your Gynecologist If She Was Your Best Friend, I was 35 so I decided to go for the amnio. I can’t say whether I would have acted on any bad news, but I just wanted to know. I didn’t want to expect a baby with normal chromosomes and get blindsided. So I opted to the amnio. But it was tough. I hated the waiting. I didn’t particularly like getting needled. And a dear friend has just lost her baby as a complication from an amnio.
Hold onto your maternity pants, because a new test is in the works, and things are about to change.
Until now, expectant mothers who wanted to be in-the-know about whether their baby had Down’s syndrome or another chromosomal abnormality had to endure some very big down sides, regardless of which method of prenatal diagnostic testing they chose.
AFP testing has a very high false positive rate -- meaning that your test comes back positive if your baby’s risk of Down’s syndrome weighs in at anything greater than 1 in 200 (meaning that your test may come back positive, even if there’s a 199 in 200 chance that your baby DOES NOT have Down’s syndrome). The only way to know the difference is to do an amniocentesis.
What few people think about when they flippantly agree to doing this test is the anxiety that comes with a false positive. For most women, about 2-3 weeks elapses from when your AFP results come back abnormal and when you get results from an amnio. And if you opt not to incur the risks of doing an amnio, you’ll have to wait 20+ more weeks until that baby is born.
Talk about a major freak out.
What makes some people even more uncomfortable than the false positive rate (meaning that the test comes back positive when your baby doesn’t have Down’s syndrome) is the false negative rate (meaning that the test comes back normal when there’s something wrong). With a false negative rate of about 5%, going this route can’t guarantee you a baby with healthy chromosomes. Though the false negative rate is low, it exists. So if you’re one of those Mamas that definitely wants to know, you’ll have to go straight to CVS or amnio because there’s a 5% chance you’ll get a normal result, when your baby actually has Down’s syndrome.
First trimester screening with nuchal translucency
Nuchal translucency scanning with first trimester blood test screening is a bit better than AFP because at least you get results earlier, but the false positive rate is still high -- and the false negative rate still exists. Once again, the only way to know for sure whether your baby has abnormal chromosomes is to pursue CVS or amniocentesis. And once again, you’re facing a potential freak out if your results come back abnormal.
CVS / Amniocentesis
Until this point, if you wanted to know for sure whether your baby has normal chromosomes, you had to endure either CVS testing or amniocentesis. While CVS testing can be performed in the late first trimester (10-13 weeks), amniocentesis requires waiting until 16 weeks, well into the second trimester.
Both are painful, invasive, carry a risk of infection, and have a 0.5-1% risk of miscarriage associated with them. Which means that there’s a decent likelihood that you could lose a perfectly normal pregnancy in the process of determining whether your baby is perfectly normal. Understandably, some just aren’t willing to take that risk.
A promising new test
Well, good news, soon-to-be and wanna-be Mommies. The well-respected British Medical Journal just reported the promising results of a study investigating a state-of-the-art new way to approach prenatal diagnostic testing. Turns out that Mom’s blood always carries just a wee bit of baby’s blood, and sensitive DNA assays can actually test the chromosomes of this baby blood.
In this study, the most robust version of the new blood test studied on 314 pregnancies detected Down syndrome in 100 percent of the cases (no false negatives) with only a 2.1 percent false positive rate (which is WAY lower than the current rate of 19-24% for the AFP triple screen or first trimester screening with nuchal translucency).
What does this mean for you?
Well, it’s not widely available yet. At $2,000 a pop -- and only in research settings -- your OB isn’t gonna be able to easily check this box off on the lab form, at least not yet. But the science and cost of DNA gene sequencing are changing so fast that experts predict that within as early as a year, this test might be widely available as yet another tool for parents who want definitive reassurance that their baby is healthy, without incurring the risks of amnio or CVS.
So stay tuned. And keep your fingers crossed.
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