Prenatal screening tests for Down syndrome

Down syndrome is a congenital disorder where a baby is born with three copies of chromosome 21 which causes mental and physical disabilities. We look at current screening tests for Down syndrome and talk about the new cell-free DNA testing for Down syndrome.


Down syndrome is a congenital disorder where a baby is born with three copies of chromosome 21. This causes mild to severe mental impairment, weak muscle tone, a shorter stature and a flattened facial profile.
No one knows exactly why this birth defect occurs. Anybody can have a baby with Down syndrome, but scientists do know that women age 35 and older have a significantly higher risk of having a child with this congenital disorder.

Mardelle Schoeman, a genetic counsellor at the Fetal Assessment Centre in Cape Town gives some insight as to what each of the tests entail:

Screening tests for Down syndrome

First trimester screening:

  • A nuchal translucency (NT) scan is typically done between 11-13 weeks gestation where the thickness of the fold at the back of the baby’s neck is measured. “The thicker the fold, the higher the chance of the baby having Down syndrome. Babies without Down syndrome can, however, also have a thick fold and the other way around,” explains Schoeman.
  • Several other markers on the scan are also evaluated at this stage. One in particular is the presence or absence of the foetal nasal bone.
  • The NT scan combined with a blood test can be done at around 10-13 weeks and measures the levels of two substances in the mother’s blood (PAPP-A and beta-hCG). This test is up to 95% accurate in detecting Down syndrome.
  • Screening for Down syndrome has recently been revolutionised with the introduction of non-invasive prenatal screening. A simple blood test on the mother can be done from as early as 10 weeks gestation. This test can detect Down syndrome with >99% accuracy. The Harmony Prenatal test is a non-invasive and poses no risk to the mother or foetus. Click here  for more info about this test.

Second trimester screening:

  • A foetal anomaly scan (detailed scan) is typically done between 19-22 weeks and can be used to look for any abnormalities suggestive of Down syndrome, including heart defects.

“Once a screening test has indicated that a pregnancy is at high risk of Down syndrome, a diagnostic test is necessary to confirm this. During pregnancy, diagnostic tests are invasive and have an associated risk of miscarriage (1% or less),” says Schoeman.
To do a diagnostic test, a sample must be obtained from the foetus. There are different methods available to do this test depending on the stage of pregnancy.

Diagnostic tests:

  • The most common method is known as an amniocentesis. A sample of amniotic fluid (water around the baby) is obtained using a needle through the mother’s tummy. Amniocentesis can be done between 16-20 weeks gestation.
  • Chorionic villus sampling (CVS) can be done between 11-13 weeks of pregnancy where a small sample of placenta is obtained to test. “The advantage of this method is the fact that you can get an earlier definitive result,” says Schoeman.
  • After 20 weeks, a cordocentesis can be done where blood can be obtained directly from the baby’s cord. Depending on the level of detail provided, results can take between three to five days or up to two to three weeks.

How will Down syndrome affect my child’s life?

  • Children with Down syndrome are intellectually disabled. This means they’ll develop slower. Their milestones like sitting, walking and talking will be delayed.
  • They can have a range of problems, including heart defects. “About half of children born with Down syndrome are born with a heart defect,” says Schoeman.
  • Children with Down syndrome are generally more prone to infections and can have problems with their hearing and vision.
  • In South Africa, children with Down syndrome mostly have to go to a special school and they will always need someone to look after them, even when they’re grown up.
  • Like with all people, each individual with Down syndrome is different and unique and will vary in their looks, personality, behaviour and abilities.

Click here for more information.


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