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Nuchal Translucency Screening Test

The nuchal translucency screening test, also called the nuchal fold scan, is often used as the first test to determine whether you are at risk of having a baby with a chromosomal abnormality, such as Down Syndrome, trisomy 13 or trisomy 18. It cannot diagnose an abnormality; it only determines if you are at high risk for an abnormality and can help you decide if you want to undergo more invasive and riskier diagnostic testing such as Chorionic Villus Sampling or amniocentesis. Unlike these diagnostic tests, nuchal translucency screening is painless and involves no risk to you or your baby.

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Babies accumulate fluid at the back of their neck (nuchal) during the first trimester. As a result of this fluid, the baby's nuchal fold increases in thickness each day of gestation until the fluid disappears around the 15th week. Researchers have established an average or "normal" thickness for each day during the three weeks you are eligible for the screening (between the 11th and 14th week of pregnancy). If your baby's nuchal fold is thicker than average, he or she is considered at a higher risk for an abnormality; if it's thinner, his or her risk is lower.

The screening uses ultrasound to first confirm your baby's gestational age and then measure the clear space in the tissue at the back of its neck using calipers. Your doctor will send the data from the screening to a laboratory where the nuchal fold measurements, as well as your baby's age and your age, are applied to a formula to compute your baby's chances of having a chromosomal abnormality, based on statistical probability.

The results of the screening will be in the form of a ratio that expresses your baby's chances for having a chromosomal defect. A normal result, also called a "screen negative," is not a guarantee that your baby is normal, but suggests that a chromosomal problem is unlikely. A "screen positive," or an abnormal result, doesn't mean that your baby definitely has an abnormality - simply that there is an increased risk. Most "screen positive" babies turn out to be normal. According to the American Academy of Family Physicians, the nuchal translucency screening test has a 64 to 70 percent accuracy rate.

A more accurate risk assessment, called the first trimester combined screening, combines the nuchal fold test with results from two blood tests that measure the levels of plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG) in your blood. Abnormal levels of these two pregnancy-related substances may indicate a problem. This combined testing has an 82 to 87 percent accuracy in detecting Down Syndrome and trisomies 13 and 18.

A relatively new option that appears to offer the most accurate assessment of fetal chromosomal and neural tube health is the combined integrated screening, or stepwise sequential screening. This test combines the results of the first trimester combined screening and the quad marker test available in the second trimester between the 15th and 20th weeks of pregnancy. Stepwise sequential screening has a 95 percent detection rate, however the results are not available until the second trimester.


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