NT Scan

Introduction and overview

Nuchal translucency is a collection of fluid under the skin at the back of a baby's neck. The nuchal translucency scan measures the thickness of this fluid to assess the risk of Down's syndrome (screening test). If the scan reveals that there is more fluid than usual, it could be a sign of Down's syndrome. A nuchal translucency scan can only indicate the level of risk of your baby having Down's syndrome. To get a definitive diagnosis, a diagnostic test such as Chorionic villus sampling or amniocentesis is necessary. These tests also carry a small risk of miscarriage, though. Therefore your doctor will only suggest them if there is a high risk of Down's. The nuchal translucency scan is usually advised in first time pregnancies, high-risk pregnancies and in pregnant women who are over 35 years of age. 

The NT scan is usually performed between 11 and 14 weeks of pregnancy. Before 11 weeks the scan is technically difficult because the baby is so tiny and, after 14 weeks, excess fluid may be absorbed by the baby's developing lymphatic system. 

The nuchal translucency scan is usually an abdominal scan. The examnination is started by measuring your baby from the top of its head to the bottom of the spine. Then the width of the NT is measured. The baby's skin will appear as a white line, and the fluid under the skin will look black. Your baby fits nicely on the screen at this stage, and you will be able to see the head and spine, limbs, hands and feet. An NT measurement of up to 2.0 mm is normal at about 11 weeks, and up to about 2.8 mm by 13 weeks and 6 days. This is because the NT normally grows in proportion to the growth of the baby. 

If the NT is increased, it does not mean there is definitely a problem. Some normal babies have increased fluid, too. Though the risk will be higher than the baby's with a NTwithin normal limits, nine out of 10 babies with a measurement between 2.5 mm and 3.5 mm will be completely normal.  As the NT increases, so does the risk.  

Usually, the ultrasound report is ready right after the scan is done. The report will be sent by fax directly to your doctor. A few days after your scan, the results of the complementary blood test are available and is computed and reported as your personally tailored compound test result. Again, Your doctor receives a fax report and will then discuss the results with you at your next appointment. The results will express the probability of your baby having Down's syndrome. They will be given in the form of a ratio. For example, 1:800 means that out of 800 women who have this risk level, one will have a baby with Down's syndrome. 

Low risk is a result where the risk is less than one in 300 such as 1:500. High risk is a result where the risk is greater than one in 300, for example, 1:150. This means that out of every 150 women who have this risk, one will have a baby with Down's syndrome. 

About one in 20 women (equalling 5%) have a high risk, and the vast majority of them will go on to have a normal baby. Even with a risk as high as 1:5, there is only one chance out of 5 that the baby will have Down's. Nevertheless, once your pregnancy has been labelled high-risk, you will probably feel anxious and perhaps unsure of what to do. 

The only way to confirm Down's is to have a diagnostic test such as CVS or amniocentesis. This decision can be very difficult, but you do not have to decide in a hurry. One advantage of the NT scan is that it is done early in your pregnancy, so it is possible to have a CVS while you are still in the first trimester. 

The nt scan is currently considered the gold standard of noninvasive tesing. A large research study involving over 100,000 pregnancies found that about 80 per cent of babies with Down's syndrome were correctly identified using this method. When combined with a blood test, the detection rate improves to about 90 per cent. However, to achieve these detection rates, it is important that the NT is measured accurately. This is why the scan should only be done by a trained professional. 

Further information:

– Practical introduction to NT screening

– Ultrasoundlink.net: Explanatory video on nt scan