Help interpreting NIPT results please! Attached.

[Elz1406]

I'm 34 weeks pregnant and was offered an amniocentesis earlier this week as a fetal medicine growth scan showed my baby has very short femurs - dropped below 1st centile for 1 scan and was at 3rd centile for this one.(ds1 also had short femurs but not quite as short so wasn't flagged as concern).

I didn't want the amnio and was recommended to have the NIPT which I got back today. Everything is low risk but there are a couple of bits I'm not sure about and can't find clarity online. Can anyone tell me what the "fetal fraction" is and whether mine looks ok? Also what is PPV? It seems to be high for trisomy 21 but low for the others. CN anyone explain what this means? My 12 week screening tests came back as 1 in 5000 for all 3 so I'm presuming these results mean the chances of chromosomal issues are pretty low? Thanks so much! X

PPV and the other percentages in that bottom table are related to the function of the test itself .

positive predictive value - is the probabilitythat following a positive test result, that the baby will truly have that specific disease. So it's saying that if you had got 'high risk' for trisomy 21, its highly likely the baby actually does have trisomy 21 (8% chance they do not have it, roughly )

if you got high risk for trisomy 13, there's almost a 70% chance the baby doesn't actually have it.

But It has come back low risk for all 3, and that's what matters , and that's great news! 😀

All low risk and a baby boy! Fetal fraction is the amount of dna from your baby detected, it's high because you're quite far into your pregnancy. When I had mine, I was only 13 weeks and my fetal fraction was 5.7%.

The fetal fraction is a measure of how much of the cell free DNA (which is what they use to do the test) found in your blood sample is from the pregnancy vs being your DNA. This needs to be above a certain threshold or they can’t do the test, 44% is a good amount.

PPV stands for positive predictive value, and it’s basically a measure of how likely it is someone with a positive test result actually has the condition being tested for, it’s a way of assessing how accurate the test is. Basically the NIPT is better at detecting Down’s syndrome compared to the other trisomies, which is why that number is higher. The figures in the table at the bottom aren’t specific to you, they are just provided for your information.

I hope the results are reassuring for you, and that the rest of your pregnancy goes smoothly!

Thanks so much for the very helpful replies - that makes a lot more sense now! Xx