Everything you need to know about screening for Down syndrome.
Screening for Down Syndrome
Both ultrasound and blood tests can be used to screen for Down syndrome. At your booking visit you will be offered a “combined screening test” in which the results of a blood test and scan are combined and given as a probability or “chance” (for example 1 in 500 or 1 in 2000)that your baby has Down syndrome. For women booking for pregnancy care after 14 weeks (too late for any scan based screening) it is also possible to have a blood test alone.
There are also newer tests available that measure the levels of baby’s DNA in your circulation at any time after 10 weeks (called Non-Invasive Pre-natal Testing or “NIPT”). Currently, there is no public funding for these newer tests.
Many couples find their screening options confusing. We are here to help you understand the options that are available to you.
What is Down syndrome?
All of us have 23 pairs of chromosomes. A child with Down syndrome has an extra twenty first chromosome (the condition is also called Trisomy 21). This is the result of a problem occurring at conception. Why it occurs is unknown. It becomes commoner with increasing maternal age. For example, at 30 years of age a woman has a one in 600 chance of carrying a baby with Down syndrome when she has screening tests at 11 to 14 weeks. By 40 years of age this chance is one in 70. Down syndrome is equally common in all ethnic groups. Children with Down syndrome have a varying degree of intellectual disability. A few will have other medical problems but the great majority of children with Down syndrome will lead fulfilling lives.
For more information about Down syndrome look at the websites listed on our Helpful Information page.
First trimester combined screening
For women booking for antenatal care during the first trimester screening for Down syndrome consists of a blood test taken between 9 and 14 weeks of pregnancy and an ultrasound scan (called a nuchal translucency scan) at 11 to 14 weeks of pregnancy.
These screening tests start with a probability based on your age. They then calculate your personal chances based on the levels of two substances in your blood and an ultrasound measurement taken from the back of baby’s neck. The two substances in your blood (called pregnancy-associated plasma protein (PAPP-A) and beta-human chorionic gonadotrophin (Beta-hCG)) tend to occur at different levels in your blood in pregnancies where the baby has Down syndrome. The thickness of the skin at the back of baby’s neck (the nuchal translucency measurement) tends to be increased in babies that have Down syndrome. By putting these measurements together your personal chances of carrying a baby with Down syndrome is calculated. Together these tests are referred to as a first trimester “combined screening test”.
Both these tests are completely safe and will tell you what your chances are that your baby has Down syndrome. They are screening tests – they will provide an accurate assessment of your chances but won’t give you a definite yes or no answer. Your results are given to you as a number or probability – for example 1 in 100 or 1 in 500. Used together as a screening test about 80 to 85 per cent of babies with Down syndrome will be picked up. It is impossible for all babies affected by Down syndrome to be picked up by screening because even for women with a low chance of having an affected baby an occasional baby will be born with Down syndrome (for example, for a chance of one in one thousand, every thousandth baby will have Down syndrome).
One purpose of the screening tests is to help women avoid amniocentesis. The chances of having a baby with Down syndrome increases with age but older women who might have considered an amniocentesis because of their age will usually still have a reassuring result from these tests.
Overall about 2% to 3% of women will opt to have an amniocentesis or chorionic villus sampling (CVS) after a first trimester combined screening test. This proportion will be slightly higher in older women because their initial chances based on their age alone will be higher.
The combined screening test will also screen for other rarer trisomies (having an extra chromosome) including trisomy 13 and 18. The nuchal translucency scan can also occasionally pick up early signs of an anatomical problem with baby.
Second trimester serum screening for Down syndrome
Women booking for antenatal care after 14 weeks of pregnancy (too late for a nuchal translucency ultrasound scan) can still have a screening blood test. This is called second trimester serum screening. This can be done at up to 20 weeks of pregnancy (ideally between 14 and 18 weeks). This test also takes your chances based on your age alone and calculates a personal chance based on the levels of three different substances checked in your blood.
This later test is not quite as reliable as the combined first trimester test. There is no need to have this later test if you have already had the earlier combined test.
First and second trimester screening for placental cells in maternal blood
A newer screening test for Down syndrome is also available privately (at a cost of about $600). This is a blood test that can be done after 10 weeks. The test measures the levels of baby’s placental DNA circulating in your blood. In all pregnancies very small amounts of “cell-free fetal DNA” can be detected in maternal blood. Higher levels of DNA related to chromosome 21 will be detected in your blood if your baby has Down syndrome. This test is sometimes called “non-invasive pre-natal testing” or NIPT. NIPT will detect more than 99% of babies with Down syndrome and is much less likely to give a non-reassuring “false-positive” result than the combined screening test. It can also detect high levels of DNA associated with trisomy 18 and 13.
You may also wish to do NIPT testing as an alternative to or in addition to the combined screening test. Some women have also had NIPT testing after a non-reassuring combined screening test result. NIPT testing may reassure them enough to not have an amniocentesis.
NIPT testing is not reliable enough to be considered a diagnostic test – not all women with a positive test will have a baby with Down syndrome. However, a negative test reduces the chances that a woman is carrying a baby with Down syndrome to well below 0.1 per cent. It is not currently part of the publicly funded national screening programme.
There are several different companies providing NIPT testing. AOC currently uses a test provided by the Victorian Clinical Genetic Services (VCGS) in Australia called PERCEPT. You can read more about the PERCEPT test by visiting the VCGS website or by downloading their patient information leaflet here.
This is a diagnostic test that will confirm if your baby does or does not have Down syndrome. It is usually done at between 15 and 17 weeks of pregnancy. A needle is passed through your abdominal wall into the fluid around baby – an ultrasound scan is used to guide the needle and make it a safe procedure for baby. A small amount of fluid is drawn off. Within this fluid are some of baby’s skins cells that can be cultured in the laboratory to check baby’s chromosomes.
It can take 10 to 14 days to get a result from this test. A more rapid test (called FISH) is available that can check for Down syndrome within 24 to 48 hours is also available but at an additional cost – the doctor performing your amniocentesis can discuss this option with you.
Amniocentesis is a quick procedure taking about 10 to 15 minutes to set up and only a few minutes to do. You are welcome to bring your partner or a support person along and should plan to have a quiet 48 hours after the test. It does have a small risk of miscarriage. For every 1000 women having an amniocentesis one woman will miscarry as a result of the procedure.
Chorionic Villus Sampling (CVS)
This is similar to amniocentesis but it can be performed a couple of weeks earlier in pregnancy (between 11½ and 14 weeks). A very small piece of tissue is obtained from baby’s placenta using a similar needle to the one used for an amniocentesis. Technically it can be more difficult to do with a slightly higher rate of miscarriage of 1 in 500. It is usually offered to women with a particularly high risk of having a baby with a chromosomal problem.
Useful sources of information
For many couples, decision-making around Down syndrome screening is straightforward and their test results are reassuring. All of us at AOC are aware that for some couples making these decisions is terribly difficult – we are very happy to talk to you by phone between appointments or arrange extra visits to clinic if you need more help in deciding what tests to have.
Down syndrome screening is optional – you do not have to have any screening if that is your preference.
We can provide you with additional written information to help guide your decision-making. You can download our information leaflet on medical tests in pregnancy here. Useful websites are also listed on our Helpful Information page.