The nuchal translucency screening test is among the newest of the many first-trimester screening tests now being offered to pregnant women. The nuchal translucency (NT) screening test — basically a specialized ultrasound — lets you know if you're at an increased risk for having a baby with a chromosomal problem such as Down syndrome. Unlike amniocentesis or chorionic villus sampling (CVS), however, the nuchal translucency test cannot give you a definitive answer as to whether your baby has a genetic abnormality, but rather it gives the statistical likelihood of one. With that information in hand, you and your practitioner can then decide if further (more invasive, but conclusive) testing, such as amnio or CVS, is necessary.
What exactly does it measure? The test focuses on a small, clear space in the tissue at the back of your growing baby's neck called the nuchal fold. Experts have found that this spot tends to accumulate fluid and, as a result, expand in size in babies who have genetic abnormalities like Down syndrome (caused by an extra copy of chromosome 21, one of the 23 pairs of chromosomes that contain our genetic code), trisomy 18 (an extra copy of chromosome 18), and trisomy 13 (an extra number 13 chromosome).
Nuchal translucency screening — which must be performed between 11 to 14 weeks of pregnancy (after that, the tissue gets so thick that it is no longer translucent, rendering the test inconclusive) — is done with a highly sensitive ultrasound machine (but like a regular sonogram, is considered safe). A sonographer will first measure your baby from crown to rump, to confirm his or her gestational age, before zooming in on the nuchal fold and measuring its thickness on the screen. That value, plus the mother's age and the baby's gestational age, will be entered into a formula that calculates the probability of a chromosomal abnormality.
Because NT results by themselves have an accuracy rate of just 70 to 75 percent (meaning that the test misses 25 to 30 percent of babies with Down syndrome), your practitioner might offer what's referred to as a combined or integrated screening, in which the NT ultrasound results are combined with one or two blood tests that measure and compare your levels of two maternal pregnancy hormones, beta-hCG and PAPP-A. By combining these blood tests with NT, the accuracy of detection rates for Down syndrome rises dramatically to between 83 and 92 percent.
One drawback to the NT screening, and even the combined screening, is the risk of false negatives and false positives — not to mention the emotional hoops of worry and waiting that you'll have to jump through. But if you're on the fence about amnio, NT screening may help you determine whether you need it. So talk to your practitioner, who may even refer you to a genetic counselor to explain all the possible outcomes and options.