18–20 week antenatal scan: being told something may be wrong
For some people the 18–20 week mid-pregnancy or 'anomaly' scan is the point at which they discover their baby may have a health condition. Many...
Screening test results can only tell you the probability or chance that the baby has a particular condition. This can make it difficult to understand. The nuchal translucency scan and blood test results are usually given as a probability, for example: “a 1 in 100 chance that the baby has Down’s syndrome”, or sometimes a percentage: “a 1% risk”.
These statistics can also be presented the other way round, as a 99 in 100 or 99% chance that the baby will not have a condition. (For a table showing ways of presenting probability see below).
18–20 week scans can sometimes provide a definite diagnosis, based on what can be seen on the scan. They could just show that further tests are needed.
For some people the test results can come back with a low probability of any health condition. This was reassuring although they understood that there was still a small chance that a condition could be discovered later, which could cause some people to worry.
One woman was given a chance of ‘1 in 1700’ after the nuchal scan, having been ‘1 in 300 or 400’ on the basis of her age alone. She later had a baby with Down’s syndrome. Looking back, finding out she was the ‘1’ made the figures seem meaningless.
Research suggests people do find it difficult to make sense of the results, in particular comparing the probability of having a baby with a condition with the probability of other things happening in their lives. It is difficult to understand the significance of an increased chance without knowing what the average chance is.
In most cases, people felt staff had done their best to explain test results clearly and thoroughly. Several still had trouble making sense of it, including a couple whose nuchal translucency scan result gave them a chance of ‘1 in 60’. This was lower than normal for their age, but still relatively high.
The member of staff who explained the results was careful not to steer them in any one direction. At the time they felt reassured, but afterwards rang a helpline for further advice before eventually deciding against amniocentesis.
Sometimes staff would be clear about saying whether or not to have further tests, usually when the results gave a low chance of the baby having a condition.
Previous familiarity with statistics helped some people, but they also said that others could understand statistics if they were presented in an accessible way.
Occasionally, staff also have difficulty interpreting the figures, as one woman expecting twins had discovered, but her general experience had reassured her that if staff were unsure they called a colleague to check. She had found it useful to have the printout of her results, but sometimes felt the more information you have the more questions it raises, which in turn places a responsibility on staff to explain.
A feeling of too much information was also described by a woman who was told at the 20 week scan that there was a 5% chance her baby might have serious digestive abnormalities, and she ended up having 5 scans in one day. She thought she might have been less anxious with less information (follow-up scans later in pregnancy had since reassured her the baby was probably fine).
Understanding test results is of course crucial for people who are told their chance of having a baby with a certain condition is high. Grasping the seriousness of what was happening to them and what the results meant was often difficult initially. Their experiences are explored further in the Deciding whether to have further diagnostic tests.
You may also be interested in our information on Screening for sickle cell and beta thalassaemia.
For further advice on how to understand your results, see our pregnancy resources.
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