Mr. Peter Thompson:

I could start with 100 women, all of whom have the same chance of having a baby with Down's syndrome. Of those, 40 just leave the room immediately and say, "I don't want any testing". Of the remaining 60, ten come back as high risk. Those women are all spoken to and offered an invasive test. Seven of those might take it up and three will decline it because they will say, "Actually, I was going down this process, but now I have to take a risk and I'm not going to take a risk over my baby". If any of the seven who have the invasive test out of the original 100 are identified as having a baby with Down's syndrome, a high proportion are likely to act on that information. It depends on what proportion one takes. Is it a percentage of the people who have the amniocentesis or is it a percentage of the general population? Therefore, that group is not representative of the general population.