Understanding The Nuchal Test Result
The result we give you can be difficult to interpret as often in life you do not come across risks that have considerable importance for such decision, making processes. After the testing, we will give you two results.
The first result or background risk is your risk for your age for having a pregnancy affected by Downs syndrome. This risk is based on how many weeks pregnant you are and how old you are at the time of the scan. This risk is what you have before the scan and blood test.
The second and final result is produced by using your starting risk based on your age and then combining it with the blood test and ultrasound scan findings.
Why do we use the terms high and low risk?
Understanding of the result has been deemed on a national basis to be high and low risk for purposes of both economical and test performance standards. The latter reason is so as we can compare the performance of different tests used to screen for Downs’s syndrome.
Nationally, there is an understanding that if your result falls into the category of 1 in 150 or lower – for example 1 in 151 or 1 in 5000 then you are considered low risk. However, if your risk falls in the category of 1 in 150 or higher – for example 1 in 10 or 1 in 100 etc then your result is considered high risk.
However, we prefer to counsel you individually about the result, as what may be high risk to one person is not high risk to another.
There are many ways to understand a result and we will list a few of them for you to read through.
Comparison of risk to other risks in pregnancy
As you are probably aware the only way in which we can definitely exclude or diagnose Downs syndrome is to perform a test where a needle is inserted into the placenta (CVS) or into the fluid around the baby (amniocentesis). However, both carry a risk of miscarriage of approximately 1% or 1 in 100.
Therefore, one way of comparing risks is to examine the risk of miscarriage from the needle test to that of you having a baby with Downs syndrome.
If your risk from the nuchal test is 1 in 300 and you wished to have an invasive test then you would be 3 times more likely to miscarry a normal pregnancy than have a baby with Downs syndrome.
In our audited practice, the majority of Downs syndrome cases are found where the result is 1 in 2 to 1 in 10. In fact if the result is 1 in2, the chances of having a baby with either a lethal abnormality or major chromosomal problem is nearly 75%.
Another comparison for understanding risk assessment is to compare your risk to occurrences in pregnancy. The following shows the risk for problems in pregnancy and other events in the same way as we produce the risk for Downs’s syndrome.
What are the options after a high risk nuchal test?
The options are to do nothing, have another ultrasound scan later in pregnancy or further tests which involve putting needles into the womb.
A detailed scan at about 18 weeks will examine the baby for any signs of organ damage or changes associated with Downs syndrome or what we call soft markers. Soft markers are findings on ultrasound which do not indicate damaged organs but are temporary findings of the way in which the baby’s tissue reflects the ultrasound beam. These findings cannot presently be added to your nuchal result. However a lot of people find comfort in proceeding with the pregnancy and not undertaking invasive testing knowing that the detailed scan is normal. If there are further problems seen at this stage then an amniocentesis can be performed if so desired.
Invasive or needle tests which will give a definite result for chromosomes
CVS or placental biopsy can be performed from 11 weeks of pregnancy. This is performed by Mike Wyldes or Chris Griffin who perform over 150 cases per year.
Amniocentesis can be performed from 16 weeks and again Mike and Chris perform over 150 per year.
As you can see the area of diagnosing Downs syndrome can be a potential minefield and careful consideration and counselling at all stages is essential for you to feel comfortable with your choice.
What if the PAPP – A was low?
A low PAPP – A increases the risk for miscarriage in pregnancy, high blood pressure and small babies but only by a few percentage points. However, this result needs to be 0.15MOM or less.
What if the BHCG was very high?
If the BHCG was more than 5 times the average then again there is a small increase in your risk for high blood pressure. The result will be greater than 5.0 MOM.
What if the nuchal was very thick?
The increase in nuchal size gives you the following risks in the table below.
