Coincidentally, I was discussing this with my mum and sister today even though we made the decision a while ago. I'm 25 weeks.
We choose not to have the NT scan or blood tests and I had to sign to say that I had refused the tests. I didn't have a particular objection to having an extra ultrasound scan or blood tests but didn't think we'd find the information useful.
We wouldn't do anything with the resulting risk estimate:
We tried for over 5 years to concieve this baby. We decided that we didn't want the extra risk of miscarriage (however small) by taking more invasive tests. We also wouldn't terminate for conditions like Downs. We hope our baby will have the good fortune to be entirely healthy but if he/she is not, then they will still be our baby and we will do our best to cope with the difficulties they may face. It's not just a case of trying again when its taken so long to concieve (we have no idea how long it would take if at all) and I just feel blessed to be pregnant.
And the indefinate risk calculation wouldn't be helpful for us:
We thought a high risk result would have us worrying, probably unnecessarily, for the rest of pregnancy. A low risk result could also give a false sense of security about our child's health when its no guarantee, our baby could be the one.
We were satisfied that as no anomalies were noted at the 20 week scan that any condition our child might have is not so debilitating that it affected the basic development of their essential organs.
I don't make any judgement on those who choose to have screening though. I appreciate that in different circumstances we may have made a different decision. Both of our mothers had amnios for subsequent children after carrying babies nearly to term who were still born (my angel-sister had Edwards - anomolies picked up at 20 week scan, mum persuaded to have an amnio she didn't want initially so that if the baby died it wouldn't need a post mortem , we don't know what my DH's lost siblings had). I understand why they made the choices they did (and both had healthy youngest children after their loses).
Although I don't disapprove of screening, I think the way that it seems to be routine is problematic because I think some people agree to the first lot of tests (perhaps thrilled they'll get an extra chance to see their baby) without really thinking about what they will do with the high risk/low risk information.
I'm also not certain why the emphasis seems to be on Downs Syndrome when it is a condition that can be lived with a good standard of health and happiness.