The Nuchal scan (or the nuchal translucency scan) is a special ultrasound scan now being used to measure the nuchal fold and neural tube at the back of the baby's neck. Current thinking is that babies with a particularly thick nuchal pad at the back of the neck are at a higher risk of suffering from heart defects, Down's syndrome or other chromosomal problems. Once the risk factor is calculated using this scan, expectant mothers will be offered a subsequent amniocentesis or other tests to confirm these suspicions.
The Scan is typically performed between 11 and 14 weeks gestation. It measures at the back of the fetus's neck, an area that appears as a black stripe down its back. This is strictly a screening test. A combination of measuring the nuchal fold and the mother's age will give insight into the possibility of Down Syndrome or other chromosomal abnormalities such as Turner's Syndrome. Researchers have found that 18% of fetuses with nuchal folds above 3mm had Down's Syndrome. The risk rose with the thickening of the fold - a measurement of 6mm meant 36 times the base risk. So, the overall risk for Down's Syndrome at 35 is around 1 in 270. If the fetus has a nuchal fold of 6mm, then its risk becomes 1 in 7.5.
Other screening tests include maternal Alpha-fetoprotein and the Triple Screen Test. Alpha-fetoprotein screening is blood test done on the mother to screen for neural tube defects or Down's Syndrome in the baby during pregnancy. At 17 weeks, the production of alpha-fetoprotein by the baby's liver is at a detectable level in the mother's blood. A high level of AFP indicates possible neural tube defects such as spina bifida and anencephaly. A low level of AFP indicates the possibility of Down's Syndrome. AFP screening has a high rate of false positive readings and should be followed with another blood test, ultrasound, and possibly an amniocentesis. Triple Screen is a blood test performed between 15 and 18 weeks gestation and measures levels of hCG (human chorionic gonadotrophin), estriol, and alpha-fetoprotein. This screening can detect Down's Syndrome and is more accurate than the alpha-fetoprotein test alone. High levels of alpha-fetoprotein combined with low levels of estriol indicate the possibility of Down's Syndrome. Because the accuracy of all of the serum tests depends on accurately knowing the number of weeks of pregnancy, a positive test should have an accompanying ultrasound to confirm the due date of the pregnancy. If the adjusted test results are still positive, then either amniocentesis or chorionic villus sampling should be done.
Diagnostic tests are amniocentesis and chorionic villus sampling (CVS). With amniocentesis a needle is passed through the mother's belly into the womb to sample fetal cells in the amniotic fluid. These cells are then sent for chromosome analysis. Perhaps the earliest genetic diagnostic screening is chorionic villi sampling, done at 10-12 weeks gestation. A needle is inserted into the abdomen and then into the placenta. A small tissue sample is removed and tested. The possible complications include bleeding, spontaneous abortion, and rupture of membranes. Risk to the pregnancy or baby is less than or equal to 0.5%. An ultrasound is used to guide the needle into the optimal position.
Locatelli A, Piccoli MG, Vergani P, Mariani E, Ghidini A, Mariani S, Pezzullo JC. Critical appraisal of the use of nuchal fold thickness measurements for the prediction of Down syndrome. Am J Obstet Gynecol. 2000 Jan;182(1 Pt 1):192-7.
Varney, H., Kriebs, J., and Gegor, C. Varney's Midwifery: 4th Edition. Jones and Bartlett Publishers. Sudbury, Massachusetts. 2004.
Legal fears over pregnancy scans
A more technical description of NFTS with photos can be found on FETALMedicine.Com