Antenatal Screening

Understanding screening test results

Understanding screening test results is not always straightforward, because they give only a risk or probability that the baby might have a particular condition. Nuchal translucency scan and blood test results are usually presented 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 be fine. (For a table showing ways of presenting risk see below).

18-20 week scans can sometimes provide a definite diagnosis, based on visible abnormalities or an indication that something maybe wrong that needs further investigation.

For some people the test results were clearly very low-risk and reassuring, although they understood that there was still a small risk something could be wrong. Some people felt there was little point in worrying about remote possibilities, while others could not help worrying or thinking their 'luck might run out' as somebody put it.

One woman was delighted her risk was 1 in 1700 after the nuchal scan, having been 1 in 300 or 400 on the basis of her age alone, but she later had a baby with Down's syndrome. In retrospect discovering she was 'the one' made the figures seem meaningless.

Research suggests people do find it difficult to interpret risk statistics and relate them to other risks in their lives. It is difficult to understand the significance of an increased risk unless you know what the background risk is for the rest of the population.

In most cases, people felt staff had done their best to explain test results clearly and thoroughly, but several still had trouble making sense of it, including a couple whose nuchal translucency scan result gave them a risk of 1 in 60. This was lower than their age-related risk, 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 were more directive in saying whether or not further tests were advisable, generally where the results clearly indicated a very low-risk.

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 print-out 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 risk 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'.

A separate Healthtalk site on 'Screening for sickle cell and beta thalassaemia and other haemoglobin variants' is also available. Here you will find more parents talking about risk and screening for inherited genetic disorders.



Table on ways of reframing risk (adapted from Antenatal Results and Choices)

Chance of

Down's syndrome
Chance of an
 
unaffected pregnancy
1 in 4 25% 3 in 4 75%
1 in 5 20% 4 in 5 80%
1 in 10 10% 9 in 10 90%
1 in 20 5% 19 in 20 95%
1 in 30 3% 29 in 30 97%
1 in 50 2% 49 in 50 98%
1 in 100 1% 99 in 100 99%
1 in 200 0.5% 199 in 200 99.5%
1 in 500 0.2% 499 in 500 99.8%
1 in 1000 0.1% 999 in 1000 99.9%



If you are given a risk assessment based on nuchal translucency scan and blood test results, it might help to compare this with the general age-related risk for other women of your age, but more important is the risk that applies to you individually whatever your age. 
 
If the result were 1 in 150 or higher this would be considered an increased chance (higher risk) and the mother would be offered further diagnostic tests.

For further advice on how to understand your results, see our pregnancy resources.

Last reviewed June 2014.
Last updated June 2014.

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