Screening For Down Syndrome
Pregnant women are routinely offered screening to see if their baby is at increased risk of Down Syndrome and other conditions. If your baby is at increased risk, you will be offered further tests to make a definitive diagnosis. Screening for Down syndrome can be done during the first trimester with ultrasound and a blood test.
Why test for Down syndrome?
This is a genetic condition resulting in some level of intellectual disability, a distinct appearance, and some health and developmental challenges.
In each cell of the human body, there are 23 pairs of chromosomes. Down syndrome occurs when a baby is born with an extra copy of chromosome 21 in their cells (Down syndrome is also called ‘trisomy 21’). This occurs randomly at the time of conception.
There are two types of prenatal tests for Down syndrome:
- Screening tests: These do not give you a definitive answer, but let you know if your baby is at increased risk of Down syndrome. Screening tests do not harm the mother or baby.
- Diagnostics test: These are very accurate, giving you a definitive answer. Diagnostic tests are usually offered to women whose babies are at increased risk, based on the result of the screening tests. A diagnostic test can increase your risk of having a miscarriage, so they aren’t routinely offered to all women.
You don’t have to undergo any tests if you don’t want to. If you have a screening test that shows your baby is at increased risk of Down syndrome, you don’t have to proceed to the diagnostic test. You and your family need to decide what is best for you, and you can discuss this with your doctor, midwife or a genetic counsellor.
There are 3 types of screening test for Down syndrome: the combined first trimester screening, the non-invasive prenatal testing (NIPT), and the second trimester maternal serum screening.
Combined first trimester screening
The combined first trimester screening test is done between 9 weeks and 13 weeks (plus 6 days) into the pregnancy to calculate the risk of a number of abnormalities, including Down syndrome. It is safe for both mother and baby. There will be a cost, but you can claim some of it back from Medicare.
A computer is used to combine results from 2 tests:
- A blood test, done between 9 and 12 weeks into the pregnancy, looks for hormonal changes that can suggest there is a problem with the baby’s chromosomes.
- An ultrasound scan, done at 12 to 13 weeks into the pregnancy, measures the thickness of fluid behind the baby’s neck, called the nuchal translucency. This is often larger in babies with Down syndrome.
These results, combined with the mother’s age, show the risk of Down syndrome. If the baby is at increased risk, you will be offered a diagnostic test, which is optional, to confirm whether or not the baby has Down syndrome.
Non-invasive prenatal testing
The non-invasive prenatal test (or NIPT, and sometimes called by brand names such as Harmony or Percept) is a newer, very sensitive form of screening for Down syndrome. It involves a simple blood test that analyses DNA from the baby that has passed into the mother’s bloodstream. The test is done after 10 weeks and is more than 99% accurate for Down syndrome.
NIPT tests are only done in private clinics and are not covered by Medicare. This type of test is most suitable for women who are at increased risk of having a baby with Down syndrome.
Second trimester maternal serum screening
Second trimester screening, sometimes called a maternal serum screen (MSS) or ‘triple test’, is done between 14 and 18 weeks into the pregnancy. It is usually offered to women who missed the combined first trimester screening test, or if it was not available where they were living.
It involves a blood test to look for hormones that could indicate the baby has Down syndrome or a neural tube defect. There will be a cost, but you can claim some of it back from Medicare.
If your baby is at increased risk, you will be offered a diagnostic test, which is optional, to confirm whether or not the baby has Down syndrome.
In addition to the MSS test, most pregnant women have a morphology scan (also known as an anomaly scan) at 19 to 20 weeks into the pregnancy to check on the baby’s growth and to see how the pregnancy is progressing. This can sometimes show signs of Down syndrome.
The only way of knowing for sure whether your baby has Down syndrome is to have a diagnostic test. Diagnostic tests increase your risk of having a miscarriage, so they are usually only offered to women at increased risk, women who’ve had previous babies with genetic abnormalities and women with a family history of a genetic condition.
Two tests can confirm Down syndrome:
- Chorionic villus sampling (CVS): A needle, guided by ultrasound, is inserted through the mother’s abdomen to take a sample of cells from the placenta. These are tested for missing, extra or abnormal chromosomes. The procedure is done between 11 and 14 weeks of pregnancy. It is not painful and takes about 20 minutes. The risk of miscarriage is less than 1 in 100.
- Amniocentesis: A needle, guided by ultrasound, is inserted into the mother’s abdomen, to take a sample of amniotic fluid. This is tested for missing, extra or abnormal chromosomes. This procedure is done between 15 and 18 weeks of pregnancy. It is not painful and takes about 20 minutes. The risk of miscarriage is also less than 1 in 100.
You will receive the results a few days after the test. Your doctor will explain the results, and you may be offered genetic counselling.
What to do with your results
It’s always difficult when you’re told that something is wrong with your baby, especially if you’re faced with a painful decision about the future of your pregnancy.
Your midwife or doctor will make sure you see the appropriate health professionals, such as a genetic counsellor, to help you get all the information and support you need to make the right choice for you and your family.
If the result is positive, you can choose to end the pregnancy, keep the baby, or place the baby for adoption.
Consider how you would care for a child with Down syndrome and how this would affect your family. Thankfully, the outlook for people with Down syndrome is usually very good.
Pregnant women are routinely offered screening to see if their baby is at increased risk of Down Syndrome and other conditions. If your baby is at increased risk, you will be offered further tests to make a definitive diagnosis.
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If your baby is at increased risk, you will be offered further tests to make a definitive diagnosis.
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