What does it mean to get a “high chance” result from a combined test?
In the NHS women are offered a screening test intended to identify if it is likely that their baby is affected by any of three specific conditions. These are Down’s, Edward’s and Patau’s syndromes (trisomy 21,18 and 13). The result will be reported as “high chance” or “low chance”. Sometimes this will be termed “high risk” or “low risk”, but this rather suggests that the conditions are bad or pose a threat. I have discussed this in a previous post, but I have lots of people contacting me who feel confused by the different language used.
The NHS has set a level of chance beyond which additional testing will be funded if you want it. So, if you have a chance of 1 in 2 up to 150, the NHS will offer additional testing. If the test suggests a chance of 1 in 151 or less, no further testing is offered.
This type of test will identify about 75-80% of babies affected by one of the three conditions, but many people will get a “high chance” result from a combined test when their baby is not affected. It is an assessment which could, very cynically, be called an informed guess. I am constantly surprised that many people believe that you somehow “have to” have this test in pregnancy. You do not.
As an example, if you have a chance of 1 in 140 reported for Downs syndrome, you will be offered more testing. How do you react to that?
Firstly it is important to understand that this is an estimate, based on your age, the measurement at the back of baby’s neck (the nuchal measurement or NT) and the levels of two hormones in your blood. All of this is based on assumptions about the chance of a woman of your age having a baby with Downs syndrome (which are in fact probably massive over-estimates). If you are 25 the age-related chance is assumed to be 1 in 1350, if you are 35 it is assumed to be 1 in 350 and if you are 45 the assumption is 1 in 35. The other factors are then used to refine this, based on how “average” the findings are. Confused? Yes, so are most people. I always go through the result line by line with people to help them understand this, but I am surprised how many times people have not been provided with this detailed breakdown until we ask specifically for it.
If the NT measurement is above 3.5mm at the first trimester scan, that is a feature which should be discussed regardless of other findings. It may be normal for your baby and of no consequence. It may also be an indicator of a chromosomal condition other that the three assessed for by the combined test or a structural heart issue. The hormone levels should be considered separately and in relation to each other. This shows why it is so important to discuss the result with an expert. In order to make good decisions, you need to know what you are considering as possibilities and what features are relevant. If you are unclear about your result please do contact me to discuss it.
Secondly, what do you know about the condition which is being suggested as “high chance”? The result may suggest Down’s, Edward’s or Patau’s, or a different chromosomal issue, or a heart issue – you need clear and up to date information so that you can make an informed decision about what that might mean for you and your baby. Only then can you decide if you want to have additional testing. You need to be able to consider what you can accept as the possible consequences of the test (for example the chance of miscarriage after an invasive test). You need to consider if the additional information which might be found would affect your feelings and actions. There are some useful links at the foot of this page.
Many people I have worked with over the last few years who have received a “high chance” result from a combined test have told me that they felt that they could not accept the chance of miscarriage after an invasive test, so that was not an option for them. Some then have opted for a non-invasive test (NIPT), understanding that the result is very accurate but not absolutely certain. They felt that it would be helpful to have greater certainty about the result so that they could make the best preparations for their baby if they did have an extra chromosome and the added difference that brings. to life. Others need certainty and feel that they could not continue a pregnancy in the knowledge that their baby faced one of these issues. For them an invasive test is an appropriate choice.
My aim is to provide clear and accurate information, supporting parents to make the choices that are right for them. This sometimes means arranging additional tests such as NIPT. All our lives are different, with different pressures, strengths and weaknesses. I do not presume to tell my clients what they should do. I am here to support you to do what is best for you, without judgement and with respect.
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Information about conditions affected babies hearts here