Prenatal Screening Tests for Down Syndrome

Should I have prenatal screening tests done for chromosomal abnormalities? Which ones? What would I do if that test revealed a high risk pregnancy?

genetic counselling Frieda

Frieda Loubser was the first genetic counsellor to complete her Masters in Genetic Counselling and internship training at the University of Cape Town.  She worked in the public sector (Groote Schuur Hospital and Red Cross War Memorial Children’s Hospital) and private practice for 8 years and now works in private practice full time.  She has a particular interest in inherited cancers, NIPT and Pre-implantation Genetic Diagnosis.  In her role, she brings patient education together with psychosocial support to help her patients and their families understand the implications of a strong family history of cancer and the options available to them.



Frieda’s colleague, Chantelle Scott has already written a post on Down Syndrome for this series which you can read here.

What you are about to read is a brilliant synopsis of the tests available prenatally and a case for why genetic counselling could be so valuable for expecting couples.

Many couples feel they were caught up with the momentum of what seemed to be “routine testing” and hadn’t had the chance to consider IF they wanted any prenatal testing for Down syndrome and WHAT testing options were available to them.  It is not necessarily easy to ask

Do we want to know if our developing baby has Down syndrome?”

as this questions naturally leads to another difficult question “What would we do if we find out our developing baby has Down syndrome?”

Testing for Down Syndrome during a pregnancy is an extremely personal matter and should never be made to seem routine.  Couples should be allowed to make their own informed choices based on their own beliefs, opinions and what they feel comfortable with (including the option not to pursue any such testing).  As other Genetic Counsellors, I see it as my role to facilitate this decision-making process for couples who want to have the opportunity to talk through their options and the implications to make a truly informed decision.

Testing for Down Syndrome has been available since the 1990’s (because it is relatively common and it is relatively easy to test for compared to other conditions). However, new ways of testing have been developed that may influence a couple’s decision to be tested.  I will briefly discuss the different testing options available and focus more on non-invasive prenatal testing as this is a new testing option.

It is important to understand the difference between a screening test and a diagnostic test.  A screening test for Down Syndrome cannot tell you for certain whether your baby has Down syndrome; rather, it tells you whether there is a low or high risk that the baby is affected.  By virtue of the meaning of “low risk” there is still as small risk that you could have a baby with Down syndrome after having a screening test and it resulting in a low risk result.  As long as a couple understands this option and what the test can tell them they might feel reassured by a screening test and decide not to pursue any further testing.  On the other hand a “high risk” result would lead to a couple needing to decide about more accurate testing.  In this setting “high risk” does not necessarily imply a high risk for Down syndrome (in many cases the estimated risk may be lower than 1%), but implies a “high enough” risk to consider more accurate testing: A woman with a result of 1 in 50 would be considered at “high” risk. The 1 in 50 means that, among 50 women with this same risk, one of them (2 percent) would have a developing baby with Down syndrome.

Screening tests for Down Syndrome include:

  • Maternal serum testing: Biomarkers in the maternal bloodstream which have been shown to correlate with a specific pattern in the majority of cases of a foetus with Down Syndrome (increased levels of certain hormones and decreased levels of others when compared).
  • Ultrasound markers: Markers refer to features that are common in individuals with Down Syndrome, but can also occur in someone who is unaffected (it can be a normal feature).  This finding on ultrasound will therefore increase the risk for Down Syndrome, but does not confirm it in any way.  The absence of such a feature does not exclude Down Syndrome.
  • Non-invasive prenatal testing/screening: DNA of placental origin (genetically similar to the cells of the foetus) floats in the maternal blood stream.  A new technique has been developed to compare the copies of genetic material to determine if there are extra copies of chromosome 21 and therefore testing for the cause of Down Syndrome by testing the mother’s blood sample.

This is a new option that in reality blurs the lines between screening and diagnostic testing as it is much more accurate than the other screening tests.  However as it is still an indirect test with the possibility of a false positive (showing a high risk, but the baby does not have Down Syndrome) especially in young women (who started off with a low risk) it is regarded as a (very good) screening test.  Another advantage is that it can be done as early as 10 weeks, which can lead to early reassurance to couples who are particularly concerned.  The biggest disadvantage of this option is the cost (about 3x the cost of the ultrasound and serum screening).  It is not covered by medical aid (as it is sent to the USA).

By comparison, a diagnostic test can tell for certain if the baby has Down Syndrome.

If a screening test results in a “high risk” result or you are at high risk of having a baby with Down Syndrome, you might consider a diagnostic testing method to confirm the diagnosis. A health care provider should help you weigh the pros and cons of these tests.

Diagnostic tests that can identify Down Syndrome include:

  • Amniocentesis: A sample of the amniotic fluid surrounding the foetus is drawn through a needle inserted into the mother’s uterus. This sample is then used to analyze the chromosomes of the foetus. This test can be performed from 16 weeks.  The test carries a small risk of miscarriage (~1%, but your foetal medicine specialist might discuss a lower risk for their setting according to their own data).
  • Chorionic villus sampling (CVS). In CVS, cells are taken from the placenta and used to analyze the foetal chromosomes. Typically performed after 12 weeks of pregnancy, this test carries a risk of miscarriage of ~ 2%.
  • Cordocentesis. In this test foetal blood is taken from a vein in the umbilical cord and examined for chromosomal defects.

Preimplantation genetic diagnosis is one option available for couples undergoing in vitro fertilization who are at increased risk of passing along certain genetic conditions. The embryo is tested for genetic abnormalities before it’s implanted in the womb.  Down Syndrome due to a translocation of the chromosomes is relatively rare, but for a couple where one parent carries a balanced translocation the risk of having a child with Down Syndrome can be very high.

genetic counselling 1

So how does a genetic counsellor help an expecting parent or couple?

Genetic Counsellors are trained to help couples navigate through the complexity of these options by helping couples to make a decision that is right for them.  By helping couples consider what is most important to them (high accuracy or a low risk or early answers) the most appropriate options can be facilitated. 

Early pregnancy can be exciting and wonderful. However, couples may also feel anxious and overwhelmed, especially if they have had a previous loss, diagnosis of a concerning condition or a close friend or relative who has been through a difficult time.  You might feel overwhelmed by reading the information above (it just isn’t a straight forward story).  In my experience, I have found that couples find the process of genetic counselling empowering.  Testing options are available, but they have no true value if they are not unravelled in a way that is meaningful to you. 

Frieda is based in Cape Town and if after reading these posts in the series you want to contact her for an appointment or to refer a friend to them, here are her contact details:

Mrs Frieda Loubser

Cape Town Genetic Counselling

BSc Hons (Genetics), US

MSc Genetic Counselling, UCT

HPCSA Registered Genetic Counsellor


t: 083 225 4289

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.