Nuchal Translucency Test For Detection Of Fetal Chromosomal Abnormalities
The nuchal translucency screening test is a prenatal test that is often used to assess whether you are at risk of having a baby with a chromosomal abnormality, such as Down Syndrome, trisomy 13 or trisomy 18 or other major congenital heart problems. This test is also called the NT or nuchal fold scan.
This test cannot diagnose an abnormality; however it determines if the baby is at high risk for an abnormality. This test measures the translucent space in the tissue at the back of the developing baby's neck. If there is any abnormality, there is more fluid accumulation at the back of their neck during the first trimester, causing this clear space to be larger than average. If the baby’s nuchal fold is thicker than average he or she is considered at a higher risk for an abnormality. This test is usually done between 11 and 14 weeks of pregnancy.
The results of the screening are generally in the form of ratio that evaluates the baby’s chance for having a chromosomal defect. The normal result, “Screen negative” does not guarantee normal baby but suggests that a chromosomal problem is unlikely to occur. On the other hand, if it is "screen positive," indicates that there is an increased risk. According to the American Academy of Family Physicians, the nuchal translucency screening test has an accuracy rate of 64 to 70 percent. This test is generally combined with a blood test known as first-trimester combined screening. The combined testing has an accuracy rate of 82 to 87 per cent.
For better accuracy, stepwise sequential screening is combined with first trimester combined screening in the second trimester between the 15th and 20th weeks of pregnancy. After this test, the doctor can decide whether more invasive and riskier diagnostic testing such as Chorionic Villus Sampling or amniocentesis is required or not.