How is a test done for Down syndrome?
Down syndrome is a genetic disorder that occurs when a baby is born with an extra copy of chromosome 21. This extra copy changes how the baby’s body and brain develop, which may cause intellectual and learning disabilities as well as physical challenges for the person as they grow.
Most parents expecting a baby are offered non-invasive screening for Down syndrome. However, it’s up to parents as to whether they want to screen or test the baby for Down syndrome — it’s a personal choice.
If the screening test is positive, parents can choose to conduct a diagnostic test to confirm the condition. At or after birth, parents can opt for chromosome testing, done from a blood sample to confirm or rule out Down syndrome.
Diagnostic tests for Down syndrome are more invasive and carry a slight risk of miscarriage. It’s recommended that you speak to your GP or specialist doctor to confirm whether or not you should get a diagnostic test.
Screening test for Down syndrome
Screening tests for Down syndrome do not diagnose the disorder, but rather indicate the likelihood of the foetus having it.
Non-invasive prenatal testing (NIPT)
A maternal blood test for Down syndrome is the first place to start. It’s called non-invasive prenatal testing, or NIPT, and can be performed at any time from 10 weeks into the pregnancy.
At 10 weeks or later, the baby’s DNA is found in the mother’s bloodstream and carries information about the risk of genetic conditions such as Down syndrome. NIPT is considered more accurate than the combined first trimester screening test (explained below).
The cost for NIPT is around $400 and is not covered by Medicare or private health insurance. If the result shows a possibility of Down Syndrome, parents may choose to or be recommended by their doctor to undergo further diagnostic testing.
Combined first trimester screening test (CFTS)
Combined first trimester screening test involves an ultrasound scan and a blood test at 11-13 (+6 weeks) into pregnancy.
The ultrasound examines the fluid-filled space at the back of a baby’s neck, called nuchal translucency (NT), and the development of the baby’s nasal bone (NB). The chance of Down syndrome is higher if the NT measurement is larger and/or if the nasal bone is not readily visible.
The blood test measures the levels of the proteins PAPP-A (pregnancy-associated plasma protein) and HCG (human chorionic gonadotropin).
Age, the NT and NB measurements and the blood test results are combined to determine the likelihood of Down syndrome.
CFST is partially rebatable through Medicare.
Diagnostic tests for Down syndrome
A diagnostic Down syndrome pregnancy test is more invasive and as mentioned above, may carry with it risks of miscarriage, but can provide a more definitive answer. It’s important to note that like screening tests, diagnostic tests, too, are optional and a personal choice of the parent.
Chorionic Villus Sampling (CVS)
Chorionic Villus Sampling, or CVS, is an invasive test that is offered usually after 11 weeks of pregnancy to determine a chromosomal abnormality. The doctor may recommend this test if the mother presents with any of the risk factors (age, genetic history, high risk screening test, etc.).
Performed by an obstetrician and sonographer, CVS involves the removal of a tiny sample of tissue, called chorionic villi, from the placenta. These villi contain genetic information about the baby.
Whilst the procedure may be uncomfortable, it’s not considered painful; the level of discomfort is typically comparable to a pap smear or cervical screening test.
Amniocentesis is another diagnostic test for Down syndrome and is performed at between 16 and 20 weeks into the pregnancy.
Similar to CVS, amniocentesis also involves passing a needle through the mother’s abdomen and into the uterus. A small amount of amniotic fluid is extracted from the sac surrounding the foetus.
Down syndrome test after birth
Post birth, a blood test taken from the baby can confirm the presence of Down syndrome.
If you or someone you know is thinking about opting in for a screening or diagnostic test, we encourage you to discuss the matter with your doctor, loved ones or family members. Undergoing a test for Down syndrome can be overwhelming, so it’s a good idea to have someone to support you before, during or after the test.