Nuchal Translucency Screening
Screening for Down syndrome in early pregnancy
Every couple hopes that they will have a healthy baby. To help couples in New South Wales reduce the chance of having a baby with a disability, we are offering a screening test during pregnancy which you may choose to have if you wish.
What is Down syndrome?
Down syndrome is a condition that results in intellectual disability of varying degrees and may cause physical problems such as heart defects, or difficulties with the sight and hearing.
About 1 in 700 babies born in New South Wales will have Down syndrome.
All women have a risk of having a baby with Down syndrome, although this risk increases with the age of the woman.
Down syndrome is a chromosome abnormality. It is caused by having 3 rather than 2 copies of chromosome number 21.
What does nuchal screening for Down syndrome involve?
The test has two parts:
- A blood test from the pregnant mother’s arm
- An ultrasound scan
There is no risk to the baby from either of these procedures.
By combining the results of the two parts of the test it is possible to identify if a pregnant woman is at increased risk of having a baby with Down syndrome. By itself the test does not tell you whether or not the baby definitely has Down syndrome.
It identifies women who should be tested to determine if their baby has the condition.
The blood test, also known as the maternal serum screening test measures the amount of two different proteins called PAPP-A and beta HCG which occur naturally in the mother’s blood during pregnancy.
A change in the level of these proteins may indicate that there is an increased risk that the baby has Down syndrome.
The ultrasound can be done by a specially trained sonologist or sonographer between 11.5 and 14 weeks of pregnancy.
It is usually performed through the abdominal wall but sometimes it is necessary to do an internal (vaginal) scan.
The ultrasound has no known harmful effects on the mother or the baby.
The ultrasound allows measurement of the amount of fluid in the skin at the back of the baby’s neck. This measurement is called a nuchal translucency.
All babies at this stage in pregnancy have some fluid in this area, but a baby with Down syndrome or another chromosome abnormality tends to have more fluid. The ultrasound also double checks
- The presence of a heart beat
- Single or multiple pregnancy
- Accurate estimation of the due date of the baby
- Exclusion of some other abnormalities of the baby
When is the test done?
The nuchal translucency test can be done between 11.5 and 14 weeks of pregnancy (ideally at 12-13 weeks).
In order to receive the results on the day of the ultrasound, the blood test must be taken at least a few days before the ultrasound, preferably at 10 weeks.
How accurate is the test?
By combining the information about a woman’s age, the results of the blood test and the nuchal translucency measurement, the test can identify about 9 out of 10 pregnancies (90%) in which the baby has Down syndrome.
This is more accurate than each test done by itself.
It is very important to realise that this test is only for Down syndrome and that a ‘low risk’ result does not ensure that the baby is free of other possible birth defects.
How long will it take to get the results?
The combined results of the two tests will be available soon after the ultrasound examination.
What if the test finds my baby is at an ‘increased risk ’ of Down syndrome?
1 in 20 women will have an ‘increased risk’ result.
This does not necessarily mean that there is a problem in your unborn baby.
It means that further tests should be considered to see if there is a problem with your baby.
Many women are choosing to follow up with non-invasive prenatal screening which is also known as cell free fetal DNA or NIPT.
But if your result is very high risk, it may be better to choose Chorionic villus sampling or amniocentesis because these tests are more comprehensive and thorough. They detect not only Trisomies 13, 18 and 21. They also detect deletions and additions of other chromosomes. These conditions are rare, but more likely if your fetus has a vern thick nuchal translucency measurement.
What if the test finds my baby is at a ‘low risk’ of Down syndrome?
19 out of 20 women tested will have a ‘low risk’ result. This means that the risk of having a baby with Down syndrome is very low but does not mean that there is no risk.
Does the test pick up any other birth defects?
Cardiac defects, skeletal defects and other chromosomal abnormalities can also be suspected.
Do I have to have both parts of the test?
No, the decision is your choice entirely, but the combination of the two tests allows a more accurate result.
Do all pregnant women have the screening test for Down syndrome?
No. The decision whether or not to be tested is entirely up to you. Here are a few questions you may want to consider before you decide whether or not to have the test:
- In what ways would Down syndrome affect my baby and my family?
- What are the chances of my baby being affected?
- Would I want to know ahead of time if there is a problem with my baby?
- Would I consider termination if the baby was affected?
- The test does not identify all pregnancys with Down syndrome, does this matter?
- If I tested at “increased risk” would I consider having an invasive diagnostic test even though there is a 1 in 300 chance of a miscarriage from this test?
How can I get more information about this test?
You can discuss any of these issues with your doctor. In addition, we offer an information and counseling service regarding this test. All of our doctors, as well as our genetic counselor Dr Janan Karatas can help you navigate the choices before you.
We can also give you
- More detailed information about the test itself
- Assistance to understand the implications of the test
- Help to interpret the results
This fetus has a thin nuchal translucency measurement and a normal nasal bone. The serum screening results were also favourable so the risk of Down syndrome was calculated to be only 1/3000.