here is some info from the paper:
Of these DS cases 110 (90.9%) were
detected at a risk >1:50, eight (6.6%) at a risk 1:50 to
1:99, two (1.7%) at a risk 1:100 to 1:149 and one (0.8%) at
a risk 1:150 to 1:199.
So 90% had a risk greater than 1:50, only 1.7% had a risk at 1:100, and these were in younger women.
Not to freak anyone out but here are the risk categories for women that got good scores:
The six DS cases that were missed
are listed by maternal age (risk result): 29 years (1:2800);
32 years (1:430), 35 years (1:710), 36 years (1:760),
37 years (1:290) and 38 years (1:580)
so you could have a great score but still have a child with down's. But obviously this was very rare and tended to happen for younger women, the test is actually better for older women. They didn't have anyone older than 40 that had ratios great than 1:100 that had a child with down's (100% detection).