A couple of months ago (at 13 weeks) we had an ultrasound and blood test that gave us a 1 in 2 chance that the baby had DS. In fact, the GP said that though the highest chance that the screening could deliver was 1 in 2 (50%); given the results she thought that a more likely result was greater than 85%. This obviously freaked us out and caused my wife in particular much heart ache.
In order to better understand what all this meant I started researching everything to do with DS testing and in doing so probably read every forum topic with '1 in 2 chance of DS' in the title. I was pleased to read of cases where screenings had provided a 50% chance of DS and the child was further tested (CVS, etc) or born to discover that they were completely healthy and without any chromosomal abnormalities (an obvious thing I know given that there is a 50% chance of no problem).
Well, we had a CVS test a couple of weeks after the screening to find out that our baby is 100% free of chromosomal problems and further ultrasounds have confirmed that the extremely thick neck fat (one of the markers for the high risk) had completely disappeared.
My wife wanted me to post our story on a few of these threads so that anyone else in a similar position to us can have more hope than perhaps they would otherwise have had given their 1 in 2 chance outcome.