Just thought I'd write this up here so the information can be found if anyone else needs it really. I didn't find too much info on our exact situation but a few threads on here did help us while we were going through things, so hopefully they'll come up when people google similar things in future.
My wife is 13 weeks pregnant, and we got our combined test results back last week for chromosomal issues. Terrifyingly, this came back with a ">1 in 2 chance" of t21 (Down's Syndrome). She'd already had one miscarriage last year, so this was pretty upsetting.
We'd decided that we wouldn't want to continue with the pregnancy if it was confirmed, which made the whole thing very stressful considering we'd told everyone a few days before and were still getting congratulations through.
We immediately went to the hospital to talk with one of the consultants, and she went through all the options (NIPT blood test, which would give another more accurate percentage result, or a CVS or amnio test, which would be conclusive, but risk 1% chance of miscarriage after the invasive test). We decided that due to the high probability, we'd go for the CVS and that was booked in (we'd have to wait a few weeks for the amnio, which was too long).
The next few days were spent with frantic research and stress, while I regretted not asking the nurse for the numbers from my wife's exact blood results so I could check which numbers had led to the result.
Chances of Down's Syndrome, Edward's, and Patau's Syndrome are calculated using a formula, based on the mother's age, her blood results, and the size of the translucent fluid behind the baby's neck at the 12 week ultrasound.
Our baby's NT neck measurement was 3.2mm (which is high, the healthy average is just over 1mm iirc). The baby was wriggling around and there was a range of results from 2.8mm up to 3.6mm, but 3.2mm was the official one they used.
The blood results are judged in terms of "MoM" (multiples of median), with 1 being average.
My wife's PAPP-A was 0.75 (low), and her HCG was a rather scary 4.93 (almost 5 times average - which explained her intense morning sickness!)
Low papp-a and high HCG, along with a large NT size heavily indicated Down's Syndrome, hence the >1 in 2 result. (If NCT is low, that indicates one of the other two chromosomal conditions.)
Annoyingly, it was never made clear whether ">1 in 2" meant closer to 50% chance, or closer to 100%. The consultant we saw first said that this chance meant that if there were 2 babies, 1 would have DS. But a doctor at the CVS scan told is that ">1 in 2" does mean "1 in 1".
The CVS scan experience was fine (my wife found the jabs a bit painful), and there was a detailed anatomy ultrasound before the procedure, with full heart examination using 3d doppler imagery. The doctor didn't find any abnormalities, and there were no heart problems. The nasal bone was visible on the scan, but the doctor found it hard to get a proper view because the baby was moving too much (an absent nasal bone at 12-13 weeks is also a potential symptom of DS). She said that they don't re-measure the NT size, and just stick with the previous official measurement. I assumed this was because it does tend to change over time, and they don't want to give too much unfair hope or despair to parents struggling with the situation.
The CVS test was on Wednesday afternoon, and we had to wait stressing until Friday afternoon for a phonecall with the results. I spent these days trawling through medical journal articles checking the real probabilities of these things, and how accurate they are. There is apparently a "5% false positive rate" for the combined test -- but I'm still not entirely sure how you can have a false positive on a test which only ever gives a statistical likelihood as a results.
As far as I can tell, 3.5mm is the size at which the NT measurement becomes worrying, and 0.75 isn't TOO low a PAPP-A number. So it was the very high HCG which was pushing up the result to a 1 in 2 chance. I actually found an online cloud version of similar calculation software (https://www.prenatalscreeningcloud.com/) which I used to play with the numbers a little to see how they changed the result to see how the weightings worked.
I kept swapping between having hope after no abnormalities were found at the scan, and it potentially being due to one high HCG number skewing the result -- and the doctor telling me that it was basically a 1:1 chance.
Was she saying that because it was actually certain and ">1 in 2" means 95%? Or did she maybe misunderstand my question? Why would they say "more than 1 in 2" if the chance was actually certain to be much higher? Did the doctor tell us she wasn't re-measuring the NT because it looked smaller and she didn't want to give us hope? Or do they really never measure it? Why hadn't they mentioned anything about 3.2mm being high at the first ultrasound? Maybe it wasn't really high at all? But they did seem a little quiet. (And on and on and on ... )
(Side-note: we only had my wife's blood numbers from the printed form we got after the CVS test. Only, it actually said her HCG was 4093 times the average. Which is clearly wrong. Either this was a typo, or they'd mis-typed the actual result the first time and that had led to her very high results from the software. I decided that it was probably supposed to be 4.93 (the decimal on the numpad is right next to the 0), and the nurse had just re-typed it out wrong on the CVS paperwork. I actually checked the official government guidelines for the software they use, and there are upper limits in place for all values. But we haven't got the full blood results back yet, so it might be that my wife is a medical marvel!)
We finally got the phonecall of the CVS when we were out, trying to keep busy (and my wife forgot her phone!) so we rang them back when we got in, and got the preliminary CVS results, which was an all clear.
No signs of Down's, Edwards's or Patau's.
Needless to say, we spent the rest of the week on an emotional high like I've never felt. It's been 4 days now since the CVS test, and so most of the 0.5-1% miscarriage risk has also passed.
As I say, I've basically written this here for posterity, in case anyone else is really struggling with their combined test results as we were, as I couldn't find many examples of super high probabilities. I know from experience that it's not much reassurance reading people worrying over 1 in 150 results while you're facing a >1 in 2.