Nuchal Translucency (NT) Scan
When can this scan be done:
12 – 14 Weeks:
The scan can be performed between 11 weeks and 4 days and 13 weeks and 6 days; however, the scan should ideally be timed between 12 weeks and 3 days and 13 weeks and 3 days. A transabdominal scan is usually performed; however, it may be required to have a closer look vaginally.
The aims of the scan:
- If a dating scan was not done, to date the pregnancy accurately and determine the estimated due date (EDD);
- If a dating scan was not done, to diagnose multiple pregnancies. Approximately 2% of natural conceptions and 10% of assisted conceptions result in multiple pregnancy;
- To diagnose major structural abnormalities. Approximately 1% of babies have a major abnormality that may be visible at this stage. If an abnormality is detected, a referral may be made to a specialist for further evaluation;
- To assess the chance/risk of a chromosomal abnormality. The most common of these are Downs syndrome (also known as Trisomy 21/T21), Edwards syndrome (Trisomy 18/T18) and Pataus syndrome (Trisomy 13/T13);
- Estimation of fetal gender. Although fairly accurate, this has to be confirmed at a later stage before going shopping.
Risk assessment for Down’s syndrome:
Everybody regardless of age, have a small chance of delivering a baby with a chromosomal abnormality. Everybody has a choice to have a risk assessment for chromosomal abnormalities.
The factors that are taken into consideration when the risk for Downs syndrome is calculated are as follow:
- The age of the mother (the older the mother, the higher the incidence of Downs syndrome);
- The measurement of fluid behind the baby’s neck (Nuchal translucency/NT);
- Presence or absence of the baby’s nasal bone;
- Presence or absence of physical abnormalities;
- Blood flow through a small vessel (ductus venosus) in the baby’s liver;
- Level of 2 proteins in the mother’s blood (free B-hCG and PAPP-A). This blood test is known as the first trimester Downs syndrome biochemistry. The blood test is ideally done at 10 weeks; however, it can be done between 8 weeks and 0 days and 13 weeks and 6 days.
With the results of the blood test included in the calculation, the detection rate for Downs syndrome is approximately 95% and without the blood test results, 80%. Not all babies with Downs syndrome will show physical signs on ultrasound and we therefor feel it is important to have the blood test done. At the end of the test a risk number is calculated. A risk number of 1:2 to 1:300 is high risk, 1:300 – 1:1 000 is intermediate risk and a number above 1:1 000 is low risk for Downs syndrome. The higher the number the less likely it is statistically that the baby has Downs syndrome.
There are other tests available that have a better than 95% detection rate for Downs syndrome. These tests are also optional.
- Non-invasive prenatal testing (NIPT) is a blood test on the mother and therefore carries no risk of miscarriage. The detection rate for Downs syndrome with NIPT is 99.8%. Unfortunately, this test is still expensive in South Africa, approximately R2 500 – R6 500.
- Invasive testing carries a small risk of miscarriage, approximately 1 in 300. The two options are CVS (chorionic villus sampling) or amniocentesis. CVS is preformed between 12 and 14 weeks and amniocentesis from 16 weeks onwards. The detection rate for Down’s syndrome is 99.99% and we therefore call it a diagnostic test.
Fetal View does not offer invasive testing. We can refer you for this test if you would like.
A baby can be proven to not have a chromosomal abnormality like Downs syndrome, but still have a physical/structural abnormality and for this reason we still recommend this scan to detect structural abnormalities as found in 1% of babies.
After the 12 – 14 weeks scan, the next specialised scan that is recommended is at 20 – 22 weeks. The baby still develops up to 18 weeks and only thereafter can an assessment be done to detect structural abnormalities. There is another list of signs/markers that we check for Downs syndrome at this stage.