Pre-natal Screening and Testing for Cardiac Defects and for Down's Syndrome
Although the risk of having a baby with Down's Syndrome increases with maternal age, babies can be born to mothers of any age (we are aware of some as young as 16), and in fact most are born to younger women as the birth rate is higher. If you have already had an affected pregnancy then you will have an increased risk in any future pregnancy even though in most instances Down's Syndrome is not hereditary (you should be offered genetic counselling to discuss your risks and options for future babies).
There are a variety of tests which can be carried out during pregnancy to give either a diagnosis or a risk value of a baby having heart problems, Down's Syndrome or both. Screening tests are non-invasive tests which give an indication of the chance of having a baby with Down's Syndrome. Diagnostic tests are tests which give a definite diagnosis – for heart defects these are also non-invasive and do not put the baby at risk, for Down's Syndrome they are invasive and carry a small risk of miscarriage.
Sadly many parents do not fully understand the screening process, and as more expectant mothers are routinely offered tests, so more couples are finding themselves in an unexpected situation facing the emotional issues and stress surrounding screening. By understanding the different tests and what they tell you, we hope that it will be easier for you to make an informed decision about your pregnancy and one that is right for you and your baby. After reading this you may decide not to have any screening tests (which is your right) or you may decide to ask for more or different tests based on your own beliefs and circumstances – the most important thing is that you obtain all the information you need to make your personal decision.
Maternal Serum Screening
Often referred to as the double, triple or Bart's test, this simple blood test is offered to all pregnant women at about 15–16 weeks into the pregnancy. The blood sample is sent to the laboratory where it is tested for certain hormones and proteins (depending on the actual test being done) and this information is used together with the mother's age, weight and how many weeks pregnant she is, to assess the chance of the baby having Down's Syndrome.
The result may be expressed as positive or negative or sometimes it is given as a risk value such as 1 in 350, which means in a group of 350 people with this same risk only one will have a baby with Down's Syndrome the other 349 will not. Generally a risk value of higher (more likely) than 1 in 250 (e.g. 1 in 200, 1 in 150) will be screen positive and further tests will be offered.
It is important to be aware that having a positive screening result does not mean that the baby definitely has Down's Syndrome just that you are considered at higher risk of it happening. Likewise getting a screen negative result, whilst indicating that you are considered at lower risk does not rule out the possibility that the baby may have Down's Syndrome.
If you have had a previous pregnancy where Down's Syndrome was diagnosed (i.e. confirmed not just suspected or high risk result) you will automatically screen positive regardless of the levels in your blood and your risk will be calculated using the results and your previous history.
Ultrasound Scan (Nuchal scan)
Ultrasound scanning is used routinely in all pregnancies to check the growth and development of the baby. It can also be used to identify certain characteristics or markers, which are often found in babies with Down's Syndrome:
- Shorter arm and leg bones (femur & humerus)
- Sandal gap between the first (big toe) and second toes
- Heart defects
- Thickening or swelling at the back of the neck
Whilst the presence (or absence) of these markers cannot diagnose or rule out the possibility of the baby having Down's Syndrome, if markers are seen they provide an indication and may be helpful in making decisions about further tests.
If any abnormality of the heart is seen, referral should be made to a specialist in fetal cardiology who will be able to provide more detailed information (see Fetal Cardiology).
These tests are not painful although most women find them uncomfortable, and you are advised to take it easy for a few days afterwards. Results can take up to 18 days to come through as the cells have to be grown before they can be checked, but some units now offer preliminary results using new techniques known as FISH and PCR which are available after a few days, although you still have to wait up to 18 days for the full results. With these tests you will know definitely if your baby has Down's Syndrome but they cannot identify all conditions nor guarantee a healthy baby.
Chorionic Villus Sampling (CVS)
This test is usually only offered in specialist centres. It is carried out at about 11 weeks and because it is invasive, it carries a small risk of miscarriage which is slightly higher than that of amniocentesis (ask the consultant or midwife of you want to know the rates at your unit).
Using an ultrasound scan, a fine needle is guided either through the vagina or the abdomen and a small sample of tissue is taken from the placenta. This is then sent to the laboratory where it is analysed and the chromosomes counted. Very occasionally the test fails to give a result as either the cells do not grow or the results are unclear - if this happens you should be offered a further test.
This test is done after 16 weeks under local anaesthetic. Using an ultrasound scan, a needle is passed through the abdomen into the womb and a sample of the amniotic fluid surrounding the baby is taken. This fluid contains cells from the baby which can be analysed at the laboratory and the chromosomes counted.
Only offered occasionally, this test involves taking a sample of the baby's blood from the umbilical cord from about 18 weeks. The same analysis is applied but results may be available slightly quicker.
If a problem is suspected from a routine scan, or you have previously had a baby with a heart defect, you will be sent to a specialist unit where they can undertake a thorough scan of your baby's heart. Although it is still very tiny, the sophisticated scanning equipment together with the experience of the staff, will ensure that as much information as possible is obtained about the structure of the heart.
If everything is fine they will be able to reassure you, if not they will be able to explain what the problem is and give you some idea of what can be done about it. Some problems are quite simple, others will mean that your baby will require surgery after birth (usually not for a few months for the defects babies with Down's Syndrome have), and the unit will be able to discuss all of this with you.
You will of course be able to ask questions, but it can be easy to forget what was said with the stress you are under, so we suggest that you ask for the name of the defect to be written down for you, and take home any drawings that may have been done by the consultant. With this information it is much easier to find out more details later on if you want to, and Down's Heart Group has simple diagrams and explanations of the problems that commonly affect our children which you can refer to.
What Should You Do?
No-one can make the decisions about testing for you, nor tell you what to do if you know the baby you are carrying has a heart defect or Down's Syndrome. You need to consider very carefully what is the right decision for you and your family, based on the information that you have gathered. You should not feel pressured into making a sudden instantaneous decision, but be given time and relevant information to help you.
Perhaps examining your feelings about the following points may help you to think through your options and come to your decision.
- Would you consider termination? If not do you want to risk a miscarriage by having diagnostic tests? Or do you feel that your 'need to know' outweighs the miscarriage risk?
- Is Down's Syndrome your biggest concern or a heart defect? Screening for heart defects is non invasive and although it can't tell you if the baby has Down's Syndrome, some heart defects are commonly found with Down's Syndrome so it may provide an indication at least.
- If you are considering diagnostic tests, is a smaller miscarriage rate most important to you or getting an earlier result (CVS is done about 5 weeks before amniocentesis).
- About 10% of mums with a positive diagnostic result for Down's Syndrome choose to continue the pregnancy and use the remaining time of the pregnancy to prepare for the arrival of their baby.
- If you screen positive you have the option of having a diagnostic test with a slightly increased risk of miscarriage or continuing the pregnancy without which might be stressful. If you do not think you could cope with either option perhaps you should think about not having any screening tests at all.
- If your baby has a heart problem this may well be picked up on a routine scan and depending on the nature of the heart defect it may be suggested that the baby could have Down's Syndrome, even if you have not had any screening tests.
- If you do not feel able to bring up a child with Down's Syndrome but cannot contemplate termination, there are always families willing to adopt babies with Down's Syndrome.
Probably the best advice anyone can give you is to go through it one step at a time and decide for yourself what options you can and cannot accept, often when looked at methodically like this some actions will naturally be ruled out and the decision becomes a little easier to make.
If you would like to talk to someone, Down's Heart Group will go through your options with you and try to answer any questions you may have from a parent's perspective. We respect your need to decide as an individual and will not try to influence your decision, nor make any judgement on your final choices, although we hope that talking to parents may give you a more realistic idea of the true potential of people with Down's Syndrome.