Basically most women having babies are in their twenties and early thirties therefore they have most of the babies with downs syndrome even though their risk factors are lower. They are also more likely (particularly those in their 20's) not to bother with the testing as they often don't realise they are still at risk.

Older mothers (over 35) are more at risk due to older eggs but they are also more likely to have the scans and amnios as they know they are at a higher risk. New research shows that women over 35 who have partners under 25 (like me) have half the Downs risk of those over 35 who have similar aged partners, even though most doctors have never heard of this research and don't take it into account.

The other interesting thing is that most women who get a high combined risk NT result will not have a DS baby, yet having a good result does not mean you wont have a baby without DS (although it seems to me a lot of people seem to think it does).

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.

It's all very complicated.