Can someone explain the way integrated test results are calculated?

[littlefrog]

I've been trying not to fret about things, but I keep coming back to this, so I wondered if someone could help.

I've had the Integrated Test (for Downs etc.) in two pregnancies now. First one (aged 32) risk came back as 1:50,000. Second (current pregnancy, age 34) came back 1:990.

Now I know that 1:990 is still a low risk, but it's 50 times the risk last time around, and it's got me worried. And perplexed. How are these things calculated?

One thing I'm wondering about is the importance of accurate dating in these tests. I was 100% confident of the doctor's dates with my first pregnancy; this time I am pretty much equally sure that their dates are 5-6 days out.

Can someone explain how it works to me?

The odds are good. A 1:100 chance basically is one in 100 babies at those odds would have something going on, so you can times that almost by 10, it is like a 0.1% chance of the baby having a condition.

The odds do dramatically drop just by your age, nothing to do with the scan or blood results, or anything else just age, even mine dropped a lot because I was 25 rather than the 18 and 23 I was with my first two.

You should speak to your midwife if you are concerned, but really, they are very good odds.

oh NJ, thank you, I do know that, and I know that I'm probably just being hormonal to worry, but DH is a worrier too, and I guess I'm going to go on fretting about this until I can understand a bit about why the results differ so much...

Do you see a consultant? You could ask them, or the MW would be able to tell you, but I'd put money on it that it's just down to your age (not calling you an old fart - honest! ).

With my first two my odds were 1:4000 and odd, with this one it was down to 1:1300 which for the sake of two years at my age I think is quite a drop, but it was solely down to age and nothing else.

no, no consultant, and I'm not due to see the MW for another month or so (23 weeks, last/first saw her at 10 )!

Maybe it's just age, but it's such a HUGE change. I mean, it's as if your risk changed from 1/4000 to 1/80! (the same 50-fold change) I don't get it, and it worries me...

You are obviously concerned about this, give your MW a ring and book an appt. she can go through it all with you and hopefully put your mind at ease.

I really hope you get the answers you need

i am in the same position as you - my last pg my risk was less than 1 in 50,000, this one it's 1 in 4,300. It's really baffling me too - I don't seem to have a midwife this time - tried ringing the hospital antenatal and they were useless and told me nothing.
I have my scan next week so if that is ok I think I will relax slightly. Btw, I was 23 with last pg, 28 now so I don't know why mine's dropped so much either....

Hello babybaby,
It's odd, isn't it. I have my scan in 10 days time, so I agree, hopefully that will be reassuring.

I would really like to know how it is that this is calculated though... I guess it's always possible that they've changed how they do their numbers.

Bump for anyone who knows about this!

Hi. Not sure if this will help but I'm currently working on a research project that addresses the 'science' behind screening (in addition to other things). We found that the same blood, analysed in different labs, can throw up vastly different risk odds - to do with slight variations in computer software/operator skill we think. Also a few days out in date can effect results, as can (odd finding, but backed up from other studies) bizarre factors like if the person taking the blood is wearing perfume??!! Plus how long the sample takes to get to lab - even a day or so delay can effect. I would say in your case as Nat says tha age factor will have some effect, as will your uncertainty about dates etc. Also if you had even a small bleed in early pregnancy this can 'throw' the figures.
Hope this helps!

Diege, that's really interesting, and fits very much with my own feeling that there is a great deal of spurious accuracy in test results (of all kinds).
As a matter of interest, when you say a 'vast difference', what sort of factor differenc are you talking about - double, quadruple, ten times?

Well, the biggest differences we saw would result in someone receiving a 'high risk' result from one lab (and possibly having an amnio) to a low risk with the same blood sample in another. Differences usually weren't so dramatic, but a significant amount of samples (20%) did yield these discrepancies. Even the less significant were still noticeable different (eg. 1 in 4000 compared to 1 in 2, 500)
I think people do assume because it's 'science' that the results are standardised and fixed - that's certainly the impression we got from most of the health professional interviewed, apart from some of the antenatal screening midwives who seemed pretty clued up (more so than many 'higher up').

Diege - can I just ask - can results be effected by size of the baby? My dates are certain but my baby is measuring on the 99th centile for head and length - could they have an effect as I'm assuming the nuchal fold would then be thicker too? (mine was 1.9 at 12 weeks)

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