Important: This is intended as general information only. It may not apply to your specific situation. It is not a substitute for medical advice. If you have a question or concern, please raise it with me at your next appointment or contact me sooner if it's urgent.
Testing for Down Syndrome
As well as all the routine pregnancy tests, there are some other tests that you can have if you wish to. The most significant of these (for most people) is testing for Down Syndrome. Don't worry too much if the information below is confusing - it's a complex topic. We will discuss testing for Down Syndrome in detail at your first visit.
Down Syndrome is a genetic condition caused by an extra (3rd) copy of chromosome number 21. A normal cell has 23 pairs of chromosomes – 46 chromosomes in total. One chromosome of each pair is inherited from each parent. A sperm or an egg has only one copy of each chromosome - 23 in total – to make a total of 46 chromosomes when they join up to make an embryo.
Trisomy 21 occurs when either the sperm or the egg (usually the egg) has an extra copy of chromosome number 21, resulting in three copies in the embryo (which then becomes a fetus and eventually a newborn baby). This causes the disease we generally know as Down syndrome, which is one of the most common causes of intellectual disability in our community. The tests for Down syndrome may also detect trisomies of chromosomes 13 or 18, which are less common.
It’s possible to have trisomy of any of the other chromosomes (for example 4 or 16) but these abnormalities usually result in first trimester miscarriages (in fact, that’s the cause of most first trimester miscarriages).
There are two ways we can test for Down syndrome:
- a screening test which tells us what a baby’s risk of Down Syndrome is
- a diagnostic test which tells us whether a baby actually has Down syndrome
Diagnostic testing can be done in two ways:
- amniocentesis which involves inserting a needle through the mother’s abdomen to take some fluid from around the baby. This fluid contains some of the baby’s skin cells.
- Chorionic villus sampling (CVS) which involves inserting a fine needle through the mother’s abdomen to take some cells from the placenta. The cells in the placenta are (usually) identical to the baby’s cells.
The cells obtained are then analysed to see if there is an extra copy of chromosome 21 (or any other chromosome). These tests are extremely accurate and provide us with a definite diagnosis – a clear yes or no. The downside is that there is a very small risk (less than 1 in 250) that the procedure itself could cause the pregnancy to miscarry.
A screening test has no risk of miscarriage but does not actually make or exclude a diagnosis of Down Syndrome. Rather, it determines whether or not a baby is at high risk of Down Syndrome. The purpose of these tests is to help you decide whether or not it is worth having a definitive, diagnostic test performed.
Many parents will start with a screening test to see what their baby’s risk is. If they have a low risk result, they may decide that that they don’t wish to take the small risk of the diagnostic test to find out for sure.If a screening test demonstrates a high risk result (usually defined as a risk greater than 1 in 300), many people will choose to go on and have a diagnostic test performed to find out for sure whether or not their baby has Down syndrome.
There are two secreting tests available:
1. Combined First Trimester Screening
This is the 'standard' screening test that we have been offering in Australia for many years. It involves taking some maternal blood to measure the levels of two hormones made by the placenta, and performing an ultrasound to measure the thickness of the back of the baby’s neck. These results and the mother’s age are fed into a mathematical formula that calculates the baby’s chance of Down Syndrome. The results are expressed as a risk of 1 in something, or the ‘odds’ that a particular baby has Down Syndrome.
For example, a baby’s risk may be 1 in 1000 (1/1000) which means that there is a 1 in 1000 chance that the baby has Down syndrome. While 1 in 1000 is a good (or low risk) result it is not an actual diagnosis. It’s not telling us that a baby does not have Down syndrome, it’s telling us that the risk is low.
The best bit about this test is that is completely safe – there is no risk that this test can cause the pregnancy to miscarry. The test is not perfect but will detect about 90% of all cases of Down Syndrome. A medicare rebate is available for some of the cost of the test. In addition, the blood tests or the ultrasound may 'accidentally' diagnose a different problem with the pregnancy.
2. Noninvasive Prenatal Screening
Recently, a new test has become available in Australia for screening for Down Syndrome. It is known as Noninvasive Prenatal Screening (NIPS). There are several different companies offering this test so there are a few different brand names but they are all equivalent. IPS involves looking for fetal DNA in the maternal circulation which is normally shed from the placenta as it grows and develops. This DNA is examined to see if there is any more material from chromosome 21 present that one would expect, suggesting that an extra copy of this chromosome is present in the fetus. It can also be used to detect extra material from chromosomes 13 and 18 although the pick up rate is not as high for these conditions as for Down Syndrome. It is not a diagnostic test so any abnormality found would still need further investigation (ie a CVS or amniocentesis).
NIPS does have a better pick up rate for Down Syndrome than standard first trimester screening (up to 99% compared with 90%) so in that sense, it is a better test. However, there are some disadvantages as well. The test is more expensive at approximately $500 and there is currently no medicare rebate available. It does not work in everyone - it relies on there being enough fetal DNA in the maternal circulation to test and, if this is not the case, the test may fail. The test only looks for trisomy 21, 13 and 18 and, in some versions of the test, loss of one X chromosome. There is no chance that this test will pick up some of the other abnormalities that can be detected with Combine First Trimester Screening. If you chose to have this test, I will recommend a detailed first trimester ultrasound as well to look for any other problems.
At the moment, we are recommending NIPS to women who have been found to be at high risk of having a child with Down Syndrome based on their first trimester screening results, their age or their past history or family history. Low risk women are usually advised to have combined first trimester screening. This may change as we gain more experience with the test and as the technology becomes more affordable. If you want to know more about this test, please discuss it with me at your next appointment.
Remember that the definition of ‘high risk’ varies from person to person. Doctors consider 1 in 300 to be the cut off between high and low risk because that is similar to the risk of amniocentesis. If the risk of the disease is 1 in 10,000, we tend to think that it does not really make sense to do a test with a risk of 1 in 250 to find out for sure. However, if the risk of the disease in 1 is 10, a 1 in 250 risk to find out for sure seems quite reasonable.
It’s fine for doctors to think so mathematically - we’re not the ones taking the risk! If you decide to have this test done, you should think about what your risk level means to you. Some people think that 1 in 1000 is a scary level of risk, while some others are quite happy to live with a risk of 1 in 100. A woman whose test is reported as 1 in 250 would be considered to be in the high risk range. However remember that, while this woman has a 1 in 250 chance of having a baby with Down syndrome, she also has a 249 in 250 chance of having a baby without the disease.
Some women may choose to have the diagnostic testing done without doing the screening test first. This may include women above the age of 37, as Down syndrome becomes more common as women age. Other women may be more prepared to take the small risk of miscarriage associated with diagnostic testing rather than live with the risk (however small) of having a baby with Down syndrome.
Remember that these tests are optional – you don’t have to have ant testing at all if you don’t want to. The tests are offered so that, if your baby did have Down Syndrome, you could have the opportunity to make the diagnosis before delivery and, if you chose to, terminate the pregnancy. If you would not want to know beforehand, or if you would not consider terminating the pregnancy, there is no need to have the test.
Choosing whether or not to have testing for Down Syndrome, and which test to have, are very personal decisions. Just as it’s completely fine for one woman to choose to go directly to having a CVS or amniocentesis, it’s also completely fine for another woman to choose not to have any testing at all. Do what is right for you.