NT SCAN / NUCHAL TRANSLUCENCY 11½ – 14 WEEKS
What is a NT Scan / nuchal translucency test?
This is a first trimester test that is appropriate if you wish to do testing for Down Syndrome. Down Syndrome is a condition that affects 1:700 babies born and is associated with varying degrees of mental retardation and other physical abnormalities.
It occurs because a baby is conceived with too many chromosomes in the cells. Though these children can be helped with special education and interventions, the condition is not curable.
Nuchal translucency testing is a “screening” test for Down Syndrome that estimates the risk of Down Syndrome as a probability, for example, 1 chance in 1000. It will involve an ultrasound between 12 and 14 weeks to look at the physical appearance of the fetus and in particular to measure the thickness of the skin at the back of baby’s neck and look for a visible nasal bone. A blood test is also done to measure certain hormone levels to see whether they are a matching “fit” for those that are often seen in Down Syndrome.
After the scan, our doctor will explain the test to you and show you the images of the ultrasound. This is an opportunity for you to ask any questions if there are things about testing that you do not understand or if there is anything else that concerns you about the well being of your baby.
What do high and low risk results mean?
The results from this test alone do not diagnose Down Syndrome but can help identify those pregnancies where tests such as the amniocentesis or CVS may be appropriate.
We regard any risk greater than 1 chance in 300 as being “high risk” and would offer more testing to these women.
Approximately 90% of Down Syndrome babies show up as being “high risk” with nuchal translucency testing. This means a small number (10%) will be missed.
Of woman with normal babies 96% will get “low risk” results but about 4% will get high risk results and often this is for an innocent reason.
Because the test does not tell us which is the normal baby with the high risk result and which is the baby with Down syndrome, we offer all women with high risk results an amniocentesis or CVS.
The report that goes out to your doctor will also quote you risks for certain other chromosomal abnormalities, but it is not a complete screen for all problems that can lead to mental retardation in the baby because these can be due to many different reasons.
With the rapid improvements in ultrasound quality, some of the structural abnormalities previously picked up at the 19 week scan can now be seen earlier. If there are concerns about a different abnormality on the nuchal scan, this will be discussed with you by our doctor but follow up scans may be necessary for a confident diagnosis.
We prefer to book this scan after 12 weeks but before 14 weeks
We will ask you to have a blood test done before coming for the scan
The test reports probability of Down Syndrome. It allows us to reassure most women that they are unlikely to be carrying a Down Syndrome fetus, but not to tell them that this is impossible.