Screening tests for Down syndrome: What you need to know

All parents hope their babies are born healthy and strong. Hearing your baby has Down syndrome – a chromosomal disorder arising at conception – can be heart-breaking.

What is 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, shorter stature, and a flattened facial profile. No one knows exactly why Down Syndrome occurs. Anyone can have a baby with Down syndrome, but scientists know that women aged 35 and older have a significantly higher risk of having a child with this congenital disorder.

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.
• Children with Down syndrome are generally more prone to infections and can have hearing and vision problems.
• 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 all people, each individual with Down syndrome is different and unique and will vary in their looks, personality, behaviour, and abilities.

Screening tests for Down syndrome There are several screening tests for Down syndrome. Mardelle Schoeman, a genetic counsellor, gives some insight as to what each of the tests entail: 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 less than 99% accuracy. The Harmony Prenatal testis a non-invasive and poses no risk to the mother or foetus.

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 the 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 detail provided, results can take between three to five days or up to two to three weeks.