Screening test results

Understanding screening test results is not always straightforward, because most give only a risk or probability that the baby might have a particular condition, whilst some can give a definite diagnosis. Nuchal 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. In other cases, they too provide a risk calculation based on ‘markers’ (physical characteristics often associated with particular conditions). If the result were 1 in 200 this would be considered higher risk and she would be offered further diagnostic tests.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.

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

If you are given a risk assessment based on nuchal scan or 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. 
 
For example, a 30-year old woman gets a result which shows her risk of having a baby with Down’s syndrome, is about 1 in 500. This means that if there were 500 people in exactly her situation, 1 woman would have an affected baby and the other 499 would not. There is a chance that she is the one, and we cannot know that without further tests, but she is much more likely not to have an affected baby. Although 1 in 500 is quite a small number, it is actually slightly higher than average for all women of her age (1 in 900) – but it is still a small risk to her. If the result were 1 in 150 in the 1st trimester or 1 in 200 in the 2nd trimester, this would be considered an increased chance (higher risk) and she would be offered further diagnostic tests.

Reasons for not having some or all antenatal screening.

Some people or couples we talked to had decided on principle that they did not want screening, because they felt certain they would not consider ending the pregnancy or did not want to be put in a position where they might have to face such a decision.

Other couples decline screening not for religious reasons or opposition to termination on principle, but rather because they felt they personally could not go through with a termination.

Sometimes people’s willingness to have screening depends on the condition being screened for. For example, some women declined screening for Down’s syndrome because they would not consider ending the pregnancy for that condition and would not take the risk of amniocentesis to get a definite diagnosis, but feel differently about neural tube problems, because her family had a history of such problems. Some people turn down screening because they believe their risks to be low.

Tests can also provide valuable information for your care during the pregnancy. However, no test can guarantee that your baby will be born without an abnormality. No test is 100% accurate and some abnormalities may remain undetected.