Regardless of all the testing available, even if you've had an amnio or CVS which shows no abnormalities you can still get a surprise on the birth day and have a DS bub.
How is that possible? The amnio and CVS are chromosomal testing and they'd notice if the extra chromosomes were in the result.
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If you get a positive result (meaning a baby has DS or one of the many other trisomies) that they test for then this is 100% that the baby has it. If on the other hand you get a "no abnormality detected" it means just that - no abnormality detected - it does not mean 100% that your baby will not have a trisomy - it means that it is extremely unlikely that it will have them that's why they use "no abnormality detected" - to cover themselves just in case something in the test did not work. Extremely rare to have that happen, but it can happen.
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SaraJane - in answer to your question about if they can detect DS at the 20 week scan. Again, it's not a simple answer. At that time the nuchal measurement will most likely have decreased and that's one of the markers for DS. If other markers are present at that time (like heart defects & nasal bone abnormalities) then they might recommend an amnio. Of course by that time it would be too late to abort, that's why the earlier testing is recommended. Some DS babies however look completely normal throughout all the scans and the parents have no idea until they are born.
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The possibility of this occuring is minute. But here is an explanation of how it could theoretically happen.
CVS take a sample of cells from the chorionic villus (the finger like projections) from the placenta and examines a number of the cells to see the chromosomes. The original cell (that creates the baby) splits and splits again and again to produce the placenta and baby.
For a baby to have Down syndrome, and CVS to show no cells containing three copies of chromosome 21 (which causes DS), a mutation must have occurred only in the cells that went to produce the baby (and not in the placenta).
If this occurred, it would also be possible for only some of the cells in the baby to have three #21s, and depending on their location would influence the 'level' of Down syndrome.
Amnio takes a sample of fluid surrounding the baby which contains cells that have been released from the baby. These cells are closer (in age and arrangement) to those in the baby that cells from CVS, and thus are a more accurate representation of the baby.
If the baby is formed by some cells that have two #21s, and some cells that have three #21s, and the amnio sample only contained released cells with two #21s, the result would come back as "normal". For the sample to contain only cells with two #21s, it is more likely that there is a higher ratio of these cells to those with 3 #21s in the baby.
The chance of a CVS or amnio reporting a false 'normal' result is extremely tiny, but i guess the possibility is still there.
k
I know a woman who had CVS and had a terribly damaged placenta with several different trisomies. She declined a termination because one of the trisomies always results in foetal death anyway and she felt she'd rather let the baby pick its time to go, as it were, but now she has a 3 year old, healthy, son. The placenta had abnormalities which the baby did not. No, i don't know how this works, presumably something to do with RNA versus DNA (Ryn where are you?!) but still a scary (for me) story. They cannot give absolutes in the tests, they can only give a most-accurate guess. Nothing is certain.
I don't know the Aus law but the upper limit for abortion in the UK is 24 weeks, though in cases where the baby will not survive anyway (fatal abnormalities like anencephaly and certain heart conditions) they will deliver it early after that time and not make any attempt to help it live.
I personally did not have the tests because i knew i would not terminate no matter what, and i do not believe intellectual knowledge can prepare one for the arrival of ANY baby, healthy or otherwise. I read the chapters in my baby books on DS and premmie babies along with all the other chapters and worked on the basis that it COULD happen to me but it probably wouldn't. Really DS doesn't mean anything certain, there are different kinds of DS, with different severities of effect. Some DS people are very handicapped, some not at all. To me it's like a test to see if they will be blonde or tall or optimistic people - it means something yes, but by no means everything about them will be about DS, if they di have it. It's a very personal decision. Having lost 2 babies i could never risk miscarriage to diagnose something that required no "action" anyway.
Bx
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This can happen more readily than the reverse ('normal' placenta, baby with chromosomal variations). The cells that form the placenta are 'older' cells than those that form the baby. These cells keep dividing to produce more cells (so the placenta grows) and this is when mutations can occur.
A 'mutation' or change can occur in these cells that does not affect the baby. So some cells in the placenta may have more or less chromosomes than the usual number. There are some changes that the lab people know occur in placenta that would not be present in the baby. Other changes, like an extra chromosome 21 found in some of the placenta cells (but not all) can make it harder for the lab people to say whether the baby may also have some or all cells with an extra 21.
In this situation, a Mum may be offered a follow up amino bc this looks at cells that come from the baby. All these changes are in the DNA. Chromosomes are made up of a long string of DNA wound tightly. When a cell divides into two new ones, one of each chromosome goes to each of the new cells. If the chromosomes don't line up properly, you can get two chromosomes of the same type going in to one new cell, and none of that type going into the other new cell.
If a change occurs in a baby cell, it is usually repaired. If the change is a serious one and not repaired, the baby will not survive.
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Wow what a positive thread. How lovely!! I met someone recently who had an amnio and was told her baby was 'fine' all clear. Her baby has mosaic Down Syndrome that was not picked up when they counted the 15 cells of the amnio. It was picked up however at birth and it only took 7 cells to find one with 47 chromosomes....so yes, it can happen.....
And yes, these tests do NOT predict in any way shape or form, what your baby will be like and how you will cope with looking after it. I bet all of us have been surprised by our parenting in one way or another now that we have children....
I would strongly recommend skipping the testing as it only creates worry....I have a friend who has just had her first baby and her pregnancy was wrought with worry as the docs (she saw at least 8 different ones) were worried about the size of her bub. Not to mention her and her hubby are not that big! ggrrrr so she spent weeks worrying about baby not growing properly, placenta maybe dying etc....wondering how she would cope with an induction blah blah blah. It was so unfair. She ended up having a reasonably good birth and her baby was on the small side but perfectly normal and healthy. I can understand that doctors have to go by numbers and that some babies are in fact saved this way but really....!!!
For what it's worth, I have a child with Down Syndrome and she's perfectly normal and healthy. She has no health issues and although she picks up things a little slower than others, she's by no means a dunce! It saddens me that people are scared of people like her - after all, she's people like usI'd love to help allay people's fears.....
Cas
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