I received my results from the combined screening for the trisomies last week, and my risk for Down's was calculated as 1:23.
I went in to the hospital to talk to the consultant, but it wasn't especially helpful, and I don't feel I have any understanding of how they actually arrive at that figure.
I understand the various factors they take into account, and for the record, here are my results:
Age: 35
NT: 2.4mm
HCGb: 1.98 mom, 2.58 corrected mom
PAPP-A: 0.3 mom, 0.49 corrected mom
So I get from this that my NT is on the high end of normal, my HCGb is about two and a half times what it should be, and my PAPP-A is about half what it should be.
But what I would like to know is - how do they use all this information to arrive at a number like 1:23?
Does each of these things give me a risk factor of 1:x, and then these are all multiplied together to arrive at a combined risk factor?
The reason I ask is because literally nobody seems able to explain how they arrive at these figures, not even my consultant. I sort of get the impression that only a handful of people in the world might know what's going on, and that what happens is that they collect all this data, someone in an office somewhere puts it into a computer program that spits out a number, but none of the medical professionals we actually come into contact with really understand how that number is arrived at or how reliable it really is.
Any thoughts?
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Antenatal tests
Down Syndrome screening results - how are they calculated?
zambean · 02/10/2016 10:42
Sorry you're dealing with this. it must be very stressful.
My understanding is that you're right. The result of each test are combined to produce an overall risk. the calculation will based on pretty complex statistical analysis, so I'm not surprised that the professionals involved can't explain the detail.
I suspect that the number of tests done = lots of base data = the calculations are very accurate these days.
But this is only a risk, not a diagnosis. Are you going to have a diagnostic test?
This is fairly detailed, but doesn't explain the calculation.
Hope you're doing okay OP. I just wanted to share my experience with my current (only) pregnancy. I'm 35 years old and 21w 4d with boy girl twins, I didn't find out it was twins until the 12 week scan. That was a big shock in itself! But then came another big shock, twin B had an increased NT of 6.1 and there was mention of hydrops too. The hospital whacked a shedload of negative information on us, we were taken to 'that room' with the flowers and the box of tissues and they clearly were expecting the worst case scenario and we were spoken to about selective reduction of the pregnancy etc. I then had the bloods taken for the combined screening and the bloods came back giving twin B a risk of 1:14 for downs and 1:30 for edwards/pattau. We were devastated. Twin A on the other hand whose NT was 1.6 had a risk of 1:38000 for Downs and 1:14000 for edwards/pattau. I however did not find out the readings from the bloods so not aware of the HCGb and PAPP-A levels. We were referred to the fetal medicine unit within 3 days and from there the consultant rescanned me and remeasured the NT as 4.4 for Twin B. Still increased but not as high as the NT the regular 12 week scan showed. Consultant said there was no evidence of hydrops at all. Made us think that perhaps the 12 week sonographer wasn't accurate at all. Anyway, we decided to have the Iona test (non invasive) and after a week, our results came through as Low Risk for all 3 syndromes, in fact low risk as in less than 1:1,000,000. So I just wanted to share this with you to give you some hope. My 16 and 20 week scan showed that Twin B's NT was back to normal. It's a hellish tough road and we have been on a roller coaster but it just goes to show that quite often these things all amount to nothing. Stay positive love 
.
It's basically an equation- they plug in your age, weight (more on that in a mo) and those results in and out pops the probability. They won't be able to explain it because it's a black box to them - it's hardcore maths 🤓
They need the weight because the bloods can vary with weight. NOT because weight itself affects the result itself, if you see what I mean.
When I was pregnant I tried to find the algorithm (hello fellow nerd!) but I couldn't. It seems to be pretty complex - here's a link to a software that does it for example. www.sbpsoftware.com/trisomy-risk-calculator.html
1:23 is still an almost 95% chance of normality.
Your next question is how reliable it is. That's complicated too. All screening tests have the possibility of false positives and false negatives. You need an actual diagnostic test for a yes/no. If we are being really pedantic, even amniocentesis doesn't guarantee that because of the risk of picking up placental mosaic cells for example. As you age, the test picks up a higher percentage of cases but also through up more false positives. For example at 30 the false positive rate is 4% but at 49 it's 67%. Link below
www.ncbi.nlm.nih.gov/m/pubmed/11702835/
The new nipt testing is fantastic - it is strictly speaking a screening test but it's very very accurate. We had it. It tests not just for Down syndrome but also for the most common chromosome abnormalities and (some versions of it like panorama) test for other common genetic glitches like translocations.
Whether you wish to progress with the pregnancy or not is of course an intensely personal call and not one that anyone other than you and your partner can make. It is valuable to know one way or another though because it means that if you do progress the team looking after you can plan for any additional support needed at birth (cardiac complications etc.)
Really hope you get the results you want.
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