No the tests don't show how severe DS would be and it's a pretty large spectrum.
However, unfortunately the information given (if any) (and generally understood) about DS tends to be pretty outdated. Children with DS are now educated - they have a right to an education. Their health problems are also treated - which again makes a huge difference to outcome.
The most likely outcome (ie the most common one) is that a child with DS will grow up to be an adult who MLD's who could cope with a supported living arrangement. Usually this involves some work. I've just passed a young lady with DS this morning who was catching a bus. I see her around a lot and she is always independent and presumably is working in some form. There was an online reality tv show made following a group of young people with LD's and I would say the people with DS in there are probably pretty typical www.the-specials.com/
But DS is a spectrum, so some will be more affected - with SLD's rather than MLD's, and will require higher levels of care. Out of the kids I know with LD's and those using services locally the ones who need the really high level of support are the one with severe autism, not those with DS. So for example one service DS1 uses which offers the highest level of support locally available to those with learning disabilities - doesn't have anyone with DS accessing the highest level of support. The few with DS who use the service at all attend the higher functioning/less support needed groups, although tbh more and more often children with DS are being educated in mainstream and attend mainstream groups - for many (not all) this is appropriate.
This is the other end of the spectrum with DS
media-dis-n-dat.blogspot.com/2011/03/ny-man-with-down-syndrome-finds-success.html and here's his website members.tripod.com/~suj_2/
That sort of ability is rare - although will perhaps become more common with education and re-working of expectations. People with learning disabilities had no legal right to an education in the UK until something ludicrous like 1980 something - can't remember exactly when, but it was shockingly recent.
My concern really when it comes to termination is that people are given the above information in some form before proceeding with the termination. Not because I feel they should be not terminating or anything - that's their choice, but because if someone makes a decision based on incorrect information then if they later find out that their assumptions were not right then this could lead to them feeling they had made the wrong choice, or had been cheated in some way.
The closest it came for me was when pregnant with DS1 we decided we would terminate for anencephaly, trisomy 13 and trisomy 18 because all were 'incompatible with life'. I then (after ds1 was born) came across a website for a boy who had trisomy 13 and who lived for three months. And to be living for three months is very different from being 100% incompatible with life. I've since found that quite a few babies with trisomy 13 do live for a few weeks/months. And that changed my choices. I was horrified and felt that had I terminated a first pregnancy for trisomy 13 in my first pregnancy and then come across this website I would have been devastated. Yes ultimately the outcome would have been the same, but I would have chosen a different path to it.
So for me change it so anyone can terminate for any reason to birth, get rid of the external 'valuing' of lives. But provide women undergoing testing proper counselling so they can make an educated decision for themselves. In fact just trust women to make the right decision for them. Don't put time limits on and don't give out of date information or just assume as a doctor that living for a few weeks is ultimately the same as being incompatible with life .