Wools, this is such a difficult decision for you to make and I really feel for you--hence the following v long post!
As you may know, I am 44 and last week had to have an amnio, at 21wks, because 'soft markers' of a fatal chromosone problem called Edwards Syndrome appeared on the 20wk scan.
I'd turned down the opportunity of CVS or amnio at my 12wk scan partly because my nuchal measurement was only 1.6 and my consultant did not push me to have any further tests, saying all the statistics were basically just invented in an office somewhere. With hindsight, however, I wish I had had the tests done at 12 weeks, for three main reasons.
One, that at 12 weeks, if you find that there is a problem so serious that you want to terminate the pregnancy (or heaven forbid, if the procedure causes a m/c), you won't have to deliver the baby. Later on that isn't an option.
Two, that had I gone ahead with the procedure and learned that the baby did not have Edwards, then when the soft markers appeared I would have immediately known there was nothing to be worried about.
Three, that by 20wks I'd felt the baby kick and was more attached to it than I had been 8 wks before. Losing a baby is devastating at any stage of pregnancy but I know it would have been far worse the further I was into it.
Obviously I could have waited, knowing that the chances of Edwards were very slim (most Edwards babies will show other symptoms) but I couldn't live with the anxiety once the prospect of a possible problem had been raised. You should bear in mind that very few 20wk scans show up any kind of problem and of those even fewer indicate a problem like Edwards which isn't compatible with life.