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Antenatal Screening

Understanding antenatal screening test results

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 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. This could cause some people to worry.

 

The results of her nuchal scan indicated a very low risk, which was reassuring.

The results of her nuchal scan indicated a very low risk, which was reassuring.

Age at interview: 23
Sex: Female
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You talked earlier about odds and did you feel that you were given enough information about how to interpret risk and probability?

Yes, I think that when we had that scan at 12 weeks, I think they said something like, if you have a 1 in 300 chance of having a baby with Down's Syndrome they usually recommend further testing. So, I had to just trust that that 1 in 300 chance was the typical number but I think that that's, I think that those are the, when the NHS actually sends you for amniocentesis is if the odds are something like that.  

So that was the marker that we went by. We got really lucky, I mean the doctor who did the scan, when she came out with our odds, she said that it was the lowest chance that she had seen in years. So we felt really reassured by that, as opposed to what the number would be to actually go and have the next step, something like amniocentesis.

 

Her blood test results were reassuringly low risk, but she knew this could not guarantee Down's...

Her blood test results were reassuringly low risk, but she knew this could not guarantee Down's...

Age at interview: 32
Sex: Female
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And did it come out low on the blood test?

Well, I mean, obviously I didn't know what the overall range was. For Down's syndrome with my first child it was 1 in 1800, and from what I'd read the risk being high was viewed to be within 1 in 250 or lower, so in that respect I felt that it was quite a way off. 

It's only sort of since my second pregnancy where the ratio was very low, 1 in 3800 or something, that I realised that it can actually go that low, and that 1 in 1500 compared with 1 in 3800 is quite a bit different.

Can you get your head round these figures?

No, not really. I mean, I think that it was from, it was pure sort of curiosity that I wanted to know.

Because these risk figures are kind of, you're given nothing much to compare against, are you?

No. Well, only the 1 in 250.

Yeah.

I suppose that was my baseline, how near to 250 is my result? And 1800 is a long way away in terms of the way it's scaled, I suppose. So in terms of sort of statistical information, I could make a rational interpretation, I suppose, of the information that was being given to me. And I realised that that still is only a chance sort of estimate, it's not absolute. 

But it was sort of a high enough ratio, or is it a low enough ratio? A high enough ratio to reassure me that the chances of the child being born with Down's syndrome, given the reliability of the tests, was generally not very likely, but I always knew that till the baby was born you wouldn't actually know.

 

Her nuchal scan results were lower risk, but she still did not feel completely reassured.

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Her nuchal scan results were lower risk, but she still did not feel completely reassured.

Age at interview: 32
Sex: Female
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I mean the big thing they talked about was about Down's syndrome screening and she did all the calculations of our risk to start with, my age and those kind of things, which was one in I don't know how many. It was quite high, it was sort of one in hundreds or something. 

And then she said "and after we've done this test and done these measurements we can now say it's one in many thousand…So what the test has shown us is that it is much less likely that the baby will have Down's syndrome than when you walked through the door." Which is really nice to know, but the caveat to that is, it's always this thing about risk, there are still some cases they don't pick up in that way. 

So you never go away feeling a hundred percent reassured, which is partly what you go for is to be reassured. And you get a degree of reassurance but not, never a hundred percent reassurance. And they were very keen to stress that they often missed, there were things they could miss at this stage.

Did you feel you got enough from them about how to understand the risks that they were presenting you with?

I think they did it as well as they can. I think understanding risk is really hard.

Did they try different formulations for...?

No, no. I mean her basic thing was you came through the door as a 1 in 200, say, and you're leaving 1 in 2000, which to me was quite meaningful. I mean it did mean it was less than when you got out of bed in the morning, if that makes sense, that day. But it still wasn't what I would call a hundred percent reassuring, and maybe there just isn't such a thing…..

Do you feel glad that you've had the screening tests that you've had? Has it been reassuring in the end?

I do, because they were okay. I think if they hadn't been okay I'm not sure how I would've dealt with the decisions that you have to make as a consequence, because they don't seem entirely straightforward, and they're all about more risk calculations. So I did find them reassuring, a bit nerve-racking, and they do make you worry. But because they came out okay then it didn't feel so bad.

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 one' made the figures seem meaningless.

 

Because her nuchal scan results reassured her she was low risk, she never expected her baby would...

Because her nuchal scan results reassured her she was low risk, she never expected her baby would...

Age at interview: 37
Sex: Female
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So you came away feeling....?

Yeah, felt great. Thought, yeah, that's very positive. I mean, I was never really worried about it. Before, it's not something, that, to be honest, I'd really spent a lot of time thinking about, because I think I was, I'm a very positive person and I tend to think, 'Oh' - this sounds awful - but at least, 'It wouldn't happen to somebody like me.' And I've just led a bit of a charmed life and, do you know what I mean? I just expect the best until the worst happens. 

So although we'd sort of talked about it, I hadn't really - oh, I suppose, as well, I was more concerned that the baby was alive and everything, and that, so that was more important, really. But it was, getting the results of those scans, the scan, I did think, "Oh great," you know? 'That's one thing less to worry about', really. 

And the fact that it was so much higher than my risk could be for my age, although it's a risk, the mathematics in your mind, with the statistics, thinks, 'Well, if my risk for my age was - I can't remember what it was - but one in three or four hundred - and this is my risk, then, you know, that's a pretty sure-fire guarantee that there's not going to be anything wrong with my baby.'

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.

 

An increased risk can be difficult to interpret, without knowing about the original baseline risk.

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An increased risk can be difficult to interpret, without knowing about the original baseline risk.

Age at interview: 44
Sex: Female
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I think the medical side of it has got to be put to people. I mean, these days we all know it's not a case where a doctor is just allowed to sort of say, 'This is what you should do'. But I think it's partly making sure that people really do know, and understand what people are saying.  

One of the problems with the whole risk business is that sometimes, you know, the risk is increased by such and such a percentage, but what was the risk in the first place? You know, are you talking of one in twenty or one in two million? 

Because actually a 0.0 something percentage increase in two million is probably not worth worrying about, whereas it might well be if it's 1 in 20.

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.

 

After being reassured at first, they found the meaning of chance statistics from the nuchal scan in...

After being reassured at first, they found the meaning of chance statistics from the nuchal scan in...

Age at interview: 43
Sex: Female
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Mother' I think the whole, so the whole story was really with the nuchal fold scan, we decided to do it and went together. And it was, and I felt it was really positive, it, the whole experience was quite positive, that they gave a lot of information about what the test actually showed you, how it worked and gave us the results and so on. 

And basically what the, what it suggested was that the risk was reduced. And the woman who did the test was very positive about the fact that it had been reduced, but without being directional about how we should treat that as a decision. 

I think she was more, I think I was kind of saying, "Well, that's really positive isn't it? That's a really good thing?" and she said, "Well, yes, I think you could say that was a good thing, yes', you know. So she was trying to, she was being supportive but without directing us obviously.

Father'  And still reminding you were still high risk. 

Mother' And still reminding us that it still means that there's a risk and these are the options from here what you can do and signposting us to amniocentesis and explaining about how amniocentesis would work.

The discussion about risk you touched on earlier I mean how clear was it to you, all these different figures that you were being presented with, was it, did you feel that you had an adequate understanding of the risk information you were being given?

Mother' It was meaningless. It was meaningless, because you never, you don't think about, risk is meaningless. What actually would've been useful would be to, say, compare it to, 'Well, what's the risk about, of me having an accident if I get in a car? What's the risk of me, you know, being knocked over as I walk down the street?' And in that context it would've meant more because you don't deal with... 

Or me falling down the stairs in the house or, there are all kinds of things that you hear about that are risks that we live with every day, that we completely live with and ignore. And so all of a sudden you had to make a decision about something based on information that you never normally use.

Father' I suppose. Although, you know, I think, as I understand it's sort of anything above 1 in 300, you know, is considered high risk. And you know, I mean, they seem quite large odds, sort of thing. And so, you know, yeah - it's like [partner] says, you know - you don't know how that compares with anything else.  

And like I say, I don't think one goes through life thinking all the time, you know, 'What's the risk of being knocked over by a bus?' or whatever. But I guess if they had said, you know, 'Well, there's a 1 in 4 risk' you probably would think...

Mother'  Then you would, that would be meaningful. But I think it was, it came out as something like 1 in 60

Father' And there were issues about giving different risks back to us by different people, and that's what [partner] got a bit upset about, wasn't it?

Mother'  Yes that was very confusing, that was really confusing that there were all kinds of risks attributed to the same thing. 

Father'  Yes, so the midwife would

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.

 

Ringing the support organisation Antenatal Results and Choices helped her think through her nuchal scan results and decide not to have further tests.

Ringing the support organisation Antenatal Results and Choices helped her think through her nuchal scan results and decide not to have further tests.

Age at interview: 43
Sex: Female
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Mother' She told me a lot about what the information meant, and explained everyone of the outcomes. And that was really good, because I said, "Well, I got this score on this," and she said, "Well, that's very good, that's well within the normal parameters, so that sounds very good." And so she helped me understand the information I'd be given. 

And I guess you just don't think about, at the time you get given the information and you think you understand it, and it's not until you go away and look at it again you realise it's just a set of numbers and you don't have a clue what they mean, or what's normal and what's not, or, you know, whether that's good or whether it's not. 

So she explained all of that to me and then I said, "Well this is how I feel," and she said, "Well, yeah that sounds perfectly sensible. That's a really, you've thought it through, it's a good decision and, you know, I wish you the best of luck." 

So it was really, really helpful to just talk to somebody else who was, who knew what they were talking about and who was calm, dispassionate, but at the same time really friendly and open and helpful. And I hadn't been able to get that, because everybody else had so much invested - you know, talking to friends, they've got so much invested in the decision. We couldn't talk to anybody else who could talk to us in that informed but dispassionate sort of way, and it was brilliant. And that was the turning point really, wasn't it?

Father'  Yeah. 

Mother'  Then, you know, I rang [partner] after I'd spoken to the woman from ARC and said, "Oh, I've just spoken to, she's so, it was so fantastic, and now it's all OK, and I...". And it was just completely plain sailing, I never gave it another thought from then on.
 

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 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.

 

The midwife explaining her scan results got the graph the wrong way round, but checked it with a...

The midwife explaining her scan results got the graph the wrong way round, but checked it with a...

Age at interview: 38
Sex: Female
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I think that there were a couple of occasions when the person didn't understand the information, in fact she got it wrong. But I have to say that - and that was a midwife - that what she did do was, she obviously wasn't confident about what she was telling me, and she went and got someone else to come along. 

Somebody more experienced. And that was a common fact, feature of the care in the hospital, that wherever anybody had any concerns they deferred to somebody with more experience. And, you know, what more can you ask for, really? If there's something that's raising a concern, they don't want to, they're not going to ignore it. 

They might have a view, and generally the view was, 'I'm sure this will be fine, but I'm just going to check'. And that has been a feature of the screening that we've had. So wherever there's ever been any question mark, they've always gone to somebody else, to back up, to get a second opinion.

Can you remember what the specifics were of what she had not understood when she was trying to explain it to you?

It was just, there's a, on the measurements of the baby, it goes from one end to the other with the average in the middle and she'd just got it the wrong way round so that above, going this way to the right was whether the baby's head was bigger than average, and she'd got it the other way round.  

Because I had one scan where one of them, I can't remember what the measurement was, but it was actually much smaller than average, and she flicked it the other way and it said it was much larger than average. I mean, and in a way that wasn't particularly a concern. It wasn't, that wasn't the problem. It was that it wasn't average, which was, you know, it was sort of at the extreme end of either way, but she had misunderstood. She got it the wrong way round.

Was it you that pointed out to her that you thought that wasn't what it was showing you or...?

Yeah, because I'd, when I'd asked before about, 'Well, how does it go? Which way does it go?' the midwife had said, 'It's from small to the large, like that', so that I knew that she'd got that wrong.

 

She had good information about the scan results but sometimes having more information raises more...

She had good information about the scan results but sometimes having more information raises more...

Age at interview: 38
Sex: Female
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That was pretty good and the information that you get from the scan is very helpful, because you get a lot of information there. You have a copy of your, the print-out, because I have my medical notes that I keep with me and so you get all the results from the scan and you can look at them and afterwards you're able to ask the midwife, 'What's the significance of this? What's that? What's the other?'.

I mean, I think it does mean, to some extent, you have to recognise that the more information you've got, the more there are potential questions that you'll worry about because, for example the information is about averages and so they're measuring your babies according to some set average. And so you have to recognise that, and keep in your mind that what, this is the average and your baby will have its own peculiarities. 

But it does mean that anything that's not average or normal or kind of in the middle, makes you think, 'Well, what's wrong with this? There's something wrong with that. Is that a problem?' And they, it's easy for them to be reassuring because they say 'It's a range, and so long as your baby's within this range then, then that's fine, there's no problem about that.' But I think that it does mean that because you've got this information, you have a need to be reassured, more so than if you never knew about it before.  

You know, if you didn't know about the head, abdomen circumference and that being a significant thing, you'd never, you'd never know. You'd just go and say, 'Oh, my baby's new. What's your baby like? Is it skinny, is it fat, or whatever, you know, is it fine?' It would be a different sort of experience but, so I think it does place a burden on the medical staff to have to be able to explain the results, because if you see anything that is slightly out of the average and not straight down the middle, then you need an explanation.

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).

 

She was impressed with the medical care in her current pregnancy, but felt overwhelmed with...

She was impressed with the medical care in her current pregnancy, but felt overwhelmed with...

Age at interview: 35
Sex: Female
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I think, maybe because I'm not that medically minded, I would have thought, 'Maybe it's a bit small, hopefully it will have grown in two months.' I'm not the sort of person who would be up on the computer sort of working out, 'What does a small stomach mean?' So I do think I was given quite a lot of information, when there was a 5, they said there was a 5% chance that it was this. 

And in my rational moments I was very sort of controlled about it and didn't think about it, but in my irrational moments and emotional moments I sort of panicked. 'What if it is this, you know? A month in hospital with three small children and a tiny baby, and juggling three different schools and my work.' You know, I just, the pressure at times just felt quite a lot.

Did anybody sit down with you before they gave you all this stuff and say, 'How much information would you like?'

No, not for one minute. It was given to me in sort of just bits and pieces, you know. No-one actually said, 'This is the worst case scenario, you know. This is what - I mean, it's very, very unlikely'. They didn't even really say that until this one leading guy drew me a picture and he said, 'With fingers crossed, smile, 95% I think it's going to be fine'. 

And at that moment I did feel a slight sense of relief, but until then, you know, I felt I was given a lot of information while you're lying on the bed being scanned. And you feel a bit like, I did feel a bit like, maybe for some people it was right, but for some people it's determining what situation they're in. Maybe with a first baby, even though it would have been completely traumatic you've got a bit more time to kind of digest it and kind of deal with it. 

In the situation that I felt I was in, it was kind of, I was more panicking about how on earth would I cope with a small baby in hospital for a month, three other children. It was like the whole situation made my mind slightly run out of control, and just sort of - and the trauma of your baby having to be taken away immediately.

Looking back over it and your other experiences of screening as well, what would you feel is the best way for health professionals to handle giving people information? Have you come across really good examples where you feel, 'Yes, that was the way I wish it had always been'?

I felt after that day I wished in the end someone had had time - just one person - like 5 minutes to say, 'These are the, this is what might happen, this is what probably won't happen, and this is the probability of it all', but just in quite a sort of calm way. 

Rather than sort of 10 different people all saying bits and pieces, and you're coming away and you can't really... But I did feel that they were incredibly thorough and given their schedules, you know, they fitted me in as an emergency and I saw everyone I needed to see in that one day.

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'.

 

When she first heard her nuchal scan results, she did not interpret 1 in 20 as a very high chance.

When she first heard her nuchal scan results, she did not interpret 1 in 20 as a very high chance.

Age at interview: 36
Sex: Female
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My husband is quite a assertive sort, and he pushed her a little bit further than I guess she wanted to go, and she told us that the result of the scan was that we had a probability of the baby having Down's of 1 in 20. So I naively said, 'Oh well that means that 95% sure that the baby, that the baby is fine.' 

So I wasn't especially worried. And she said, 'No, no, no, my dear, that's quite high. That is something to worry about'. So we were sent off with our scan report. And in fact we didn't see a doctor. We saw a midwife, who went through the results of the scan with us and explained that 1 in 20 was a very high risk, and that she wanted to refer us up to a specialist hospital where they would carry out further investigations.
 

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.

Last reviewed July 2017.
Last updated June 2014.

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