Just got the news that the baby is chromosomally healthy and a girl!!! yay!! but now I'm wondering, how do they know they isolated fetal cells and not maternal cells? I mean, I'm a chromosomally healthy girl, is it possibly they just got my data and it was too early to get the fetal cells? I tested at 10w3d. How would they know? Has anyone ever heard that the NIPT got the gender wrong??
I called a left a message with the OB nurse but maybe someone here knows...
pomegranate / 3895 posts
Okay, so I did an insane amount of research on this. The odds of it being wrong are really, really slim. It depends on the exact test you took, some have higher margins but still like 5% max. Your results should have data on how much (% wise or something) fetal DNA they could isolate. That provided me a lot of comfort. I flipped out because I was 10 weeks 1 days and the results came back (in my mind) way too fast.... but it's been verified by ultrasound like 3 times (with an aside that ultrasounds are less accurate regardless of test type).
Full disclosure, I didn't buy anything until after my anatomy scan.
pomegranate / 3521 posts
I'm not sure if the exact science but the margin for error is very low! Congrats on your girl 
honeydew / 7463 posts
If they can isolate your DNA isn't it easy for them to differentiate the baby's DNA? Like if they know yours and find one that's different, it's obviously the baby's?
But this is my thought process, no science or research there.
coconut / 8472 posts
Basically if they don't find any Y chromosomes you can assume girl. Most of the tests today are 99% accurate. I'm not sure how they know they got a good sample, but they do tell people when there's not enough fetal cells.
pear / 1930 posts
As far as I know, the tests look for cells from the placenta in your blood and analyze those. They can tell they are placenta cells vs. your normal blood cells.
grapefruit / 4321 posts
@LBee: I actually know someone who's NIPT was wrong! She had a boy first and apparently some Y chromosomes can hang around inside mom so they found Y chromosomes for her second pregnancy and told her it was another boy. Anatomy scan (and delivery) proved otherwise. Isn't that insane??
grapefruit / 4361 posts
@Truth Bombs: that is SO weird. I was reading about fetal DNA and one of the things I read said that fetal DNA mostly disappears even within just 2 hours of the birth!!
pomegranate / 3895 posts
@Truth Bombs: Yet you still told me to paint all the things pink. I know you just wanted T's clothes.
pomelo / 5129 posts
I wonder if they can tell by the amount? The baby's DNA (that they examine) is usually like 10-20% (my results said how much there was). I wonder if the majority is always mama's
pomelo / 5084 posts
@DesertDreams88: The only errors my RE told me about were where someone did IVF with two embryos where one developed and one did Not. The test can pick up the other embryo's DNA. Otherwise she told me the test is almost always accurate!
pomelo / 5257 posts
I also know someone whose NIPT test was wrong, but it was basically for the reason @wrkbrk: said. She did IVF with two embryos and lost one early on. She was told she was having a boy and then at 20 weeks, surprise! It was a girl. The doctors said the lost twin likely was a boy and that's why there was male DNA present.
pomelo / 5257 posts
Oh also, I'm not sure that they distinguish maternal from fetal DNA. I thought that they were looking for chromosomes. If there are Y chromosomes present, then you're having a boy, because the mother wouldn't have Y chromosomes.
grapefruit / 4455 posts
I don't know any more on the science behind it than has already been said but mine I think was taken at 10w4d? And it was correct with the girl, we just verified it on the anatomy u/s. Congrats on your girl!! 
clementine / 756 posts
They are able to measure the fetal fraction of DNA by looking for individual SNPs that are different between mom and baby and measuring the relative amount of each. Mom will always be the higher fraction.
The test is highly accurate, but it is just a screening test. But it is more likely to be wrong if it is positive than if it is negative. That's how screening tests are set up. So a normal girl result is very likely to be accurate. If someone gets a positive result, the actual risk for a chromosome abnormality can be anywhere from 10% risk to 90+% risk and should always be confirmed by amniocentesis.
So feel good (but not 100% certain) about a normal result, but don't completely freak out about a positive.
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