Down syndrome is one of the most common genetic problems that can occur in pregnancy. If a pregnancy has a genetic abnormality it will usually be miscarried early. However Down syndrome does not always miscarry. This is why tests have been developed to check for Down syndrome during pregnancy.
People often think of Down syndrome screening as a routine test but having screening for Down Syndrome can mean you need to think about further testing and sometimes you might also get the diagnosis of Down syndrome. A clear understanding of the screening test and the consequences of a positive or negative result before you decide to have the screening ( or not) will help with your decision making along the way.
Electing to have screening for Down syndrome is a very personal decision for you and your partner. This information sheet is to help you make an informed choice about whether or not to have Down syndrome screening.
What is Down syndrome?
Every cell of our bodies contains 21 pairs of chromosomes. One chromosome from each pair must come from our father and one chromosome must be from our mother. These chromosomes carry thousands of genes in DNA which contain all our genetic information.
Down syndrome occurs when an individual has an imbalance of this genetic material because of the presence of an extra chromosome in the number 21 pair. This imbalance occurs at the time of conception, when the egg is fertilised by the sperm to form the embryo.The extra chromosome 21 comes either from the egg or the sperm so that the embryo formed has now 3 copies of Chromosome 21 instead of two. Hence the other name for Down syndrome is Trisomy 21.
Because of this genetic imbalance, babies with Down syndrome can have a range of problems. They will have intellectual disability of varying and unpredictable degrees of severity. They may have some physical disabilities, which commonly include abnormalities of the heart, kidneys and bowel. Babies with Down syndrome commonly have a characteristic facial appearance.
Why does Down Syndrome occur?
In Victoria, approximately 1 in 700 babies will be born with Down syndrome. It occurs because either the egg or the sperm involved in the formation of the embryo carried an extra copy of Chromosome 21.
The vast majority of cases (about 90%) of Down syndrome are due to a problem with the egg. A small percentage of all eggs (and sperm) have abnormal chromosome arrangements. As a woman ages, the proportion of eggs with abnormal chromosomes (compared to normal eggs) increases, so the risk of Down syndrome increases with age. However Down syndrome can occur in pregnancy in women of any age.
The remaining 10% of cases of Down syndrome are due to abnormal arrangements of chromosomes within one or the other parent, such that all their cells, not just their eggs or sperm are affected.
Can I tell if my baby will have Down syndrome?
The only way to be certain that a baby does or does not have Down syndrome is to check the genetic information within cells from the baby or the placenta. Currently the most accurate way to do this is by either an amniocentesis, which is when a needle is used to collect some amniotic fluid from around the baby (which has fetal cells floating in it), or by a chorionic villous sampling ( CVS) when the needle collects some placental tissue (which develops from the same cells as the baby). Because these tests both involve passing a needle into the uterus they are commonly called “invasive tests”.
CVS is done between 11-14 weeks and amniocentesis is done from 15-16 weeks of pregnancy. While these tests will give definite answers, they carry with them a small risk of complication, including miscarriage, of approximately 1 in 150- 200.
Because of the risk associated with these procedures, screening tests have been developed to allow people to get an idea of the risk of Down syndrome for their pregnancy and whether they may even avoid an invasive test.
Screening tests for Down Syndrome
A Down syndrome screening test is a test that does not carry any risk to your pregnancy. It collects information about your baby, and gives a guide as to how likely it is to have Down syndrome in the current pregnancy. This is expressed as a “risk of Down syndrome”. However, it is not 100%, so it does not tell you that a baby does or does not have the condition definitively. This is important, as Down syndrome can still occur (although not often) with a “low risk” result, and conversely, people with a “high risk” result will not always have a baby with Down syndrome.
There are currently 2 good screening tests for Down syndrome, the combined first trimester screening test ( CFST) and the non-invasive prenatal test ( NIPTS). These are available to all women.
The Combined First Trimester Screening Test (CFTS)
The CFTS is made up of a blood test, an ultrasound and the egg age. The blood test measures the hormones PAPP-A and HCG. It needs to be done between 9 and 12 weeks (10 weeks is the best time). The ultrasound measures the length of the baby, the back of the baby’s neck (nuchal translucency), the nasal bone and the blood flow in baby’s blood vessels. It needs to be done between 11+3 weeks and 13+6 weeks. The egg age is usually the mothers age unless the pregnancy is following IVF. The blood test results are held in the lab until the scan is completed, and then the results are combined, so you receive one result only.
The CFTS puts information from your pregnancy into a mathematical formula : Down syndrome risk = blood hormones( 10 weeks) + ultrasound ( 12 weeks) + egg age
Your test result is compared to that expected in a normal pregnancy and in a Down syndrome pregnancy and then your baby is characterized as either more likely to be a normal or more likely to have Down syndrome.
The result is given as a “risk ”number. By convention, any result that is “worse” than 1 in 300 is classified as “high risk” (eg 1 in 100, 1 in 50, etc) and any result that is “better” than 1 in 300 is classified as “low risk (eg 1 in 500, 1 in 1,000 etc)”.
If the test is low risk, it will identify a normal pregnancy correctly 85% of the time.
95% of women having CFTS will be given a “low risk” result and while the risk of having a baby affected by Down syndrome is very low, it is never zero, and very rarely a baby will be born with Down syndrome following a “low risk” result.
About 4-5% of women screened with the CFTS will be given a “high risk” result but most of those women will in fact not have a baby with Down syndrome. They will need to think about if they want further testing.
The cost of the CFST is partially rebatable through medicare.
Free Fetal DNA or Non Invasive Prenatal Testing (NIPT)
NIPT is the newest screening test available and offers a very sensitive way to screen for Down syndrome. It is performed from a blood sample taken from the mother at, or after, 11 weeks of pregnancy.
NIPT is based on looking at genetic material that comes directly from the baby. During pregnancy, some of the baby’s DNA crosses into the mother’s blood stream. DNA carries genetic information on chromosomes. The DNA mainly comes from cells shed from the placenta which, as the placenta is formed from the same original cells as the baby, is the same as the baby’s DNA. This DNA is called “Free Fetal DNA (fFDNA)”. Free fetal DNA can be isolated from the mothers blood by the ninth week of pregnancy.
Using this fFDNA it is possible to identify common chromosomal problems in the baby. Currently we look for Down Syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), Patau syndrome (Trisomy 13) and sex chromosome abnormalities. This test is known as Non Invasive Prenatal Testing (NIPT). Unlike the CFST, which reports the result as a number risk, the NIPT result is reported as just either “low risk” or “high risk”.
A low risk result will identify a normal pregnancy correctly 99.95% of the time. The test can also tell you the sex of your baby if you wish to know.
NIPT can identify more than 95% of pregnancies affected by Down Syndrome, Trisomy 18 and Trisomy 13 and 92% of pregnancies with sex chromosome abnormalities. Just as importantly, the NIPT has a very low rate of giving an incorrect result, that is of incorrectly saying the pregnancy is abnormal when it is not or of saying that it is normal when it isn’t. This means that women are less likely to have an unnecessary CVS for a high risk result.
NIPT a very good screening test but it is not 100%. This means that if the test result is “high risk” it still needs to be checked using an invasive test (either CVS or amniocentesis).
If the NIPT blood test is taken at 11weeks, the result is usually available in 3-5 days which is in time for CVS to be performed at 12-13 weeks if needed.
There is is a small chance (2%) that no result can be obtained with NIPT the first time and the test will need to be repeated. You will not be charged for a repeat test.
Although the 12 week scan is not needed for the NIPT, it is recommended that a first trimester scan is still performed for checking dating and for other early abnormalities at 13-14 weeks.
At the current time there is no medicare rebate for NIPT which costs around $400.00
IInvasive Tests: Chorionic Villous sampling (CVS) and Amniocentesis
CVS is performed between 11-14 weeks and involves taking a small of the placental tissue.
Amniocentesis is performed from 16 weeks and involves taking a sample of the amniotic fluid.
An invasive test may be considered for any of the following reasons:
- a high risk result on a screening test
- a low risk result on screening test but you are not reassure
- for you, 100% confirmation of genetical normality is worth the procedure risk
- there is another reason for doing an invasive test; for example if there is a risk of another genetic abnormality or an abnormal finding on the ultrasound
Both tests involve passing a needle through your abdomen into the uterus. Although uncomfortable, most women do not find them more uncomfortable than a blood test.
As a general statement, both tests provide the same information, although there are some very subtle differences between them, which we will discuss if you are considering the option of invasive testing.
The test performed on CVS or amniocentesis examine all the baby’s chromosomes at a microscopic level. It will confirm if Down syndrome is present (or not) very accurately. Because all the chromosomes are checked, sometimes other unexpected results are found as well.
The most important risk associated with CVS and amniocentesis is the risk of pregnancy loss (miscarriage). This is reported to occur approximately once in every 150 to 200 procedures, and is roughly the same for both CVS and amniocentesis. The most usual way for pregnancy loss to occur is after rupture of the membranes or infection. It is important to be aware that pregnancy loss can occur even if the procedure appears to have been uncomplicated.
It is important to remember that not all genetic conditions will not be detected with these tests. If you have concerns about a specific genetic condition it is important that you discuss these with me prior to having your test performed.
Why would I have testing for Down syndrome?
The decision to undertake any testing during pregnancy is very personal. Deciding whether or not to have a test for Down syndrome will be influenced by what you would do with the information. Down syndrome is not a condition that can be cured. It is not (usually) one that runs in families. A screening test for Down syndrome is offered and available to you so that you might have some measure of the risk that this pregnancy has for Down syndrome. Based on this risk, if you can decided whether to have an invasive test or not. Some women will decide not to have any screening or testing as the results would not change what they decided to do during pregnancy. Some women will decide they do want screening and testing for a high risk screen result because it would change what they will do. This information is provided to help you make the decision which is best for you.