Here is what I would have said in my reply to this woman, given more than 140 characters:

“Obviously the choice would be yours. For what it’s worth, my own choice would be to abort the Down fetus and, assuming you want a baby at all, try again. Given a free choice of having an early abortion or deliberately bringing a Down child into the world, I think the moral and sensible choice would be to abort. And, indeed, that is what the great majority of women, in America and especially in Europe, actually do. I personally would go further and say that, if your morality is based, as mine is, on a desire to increase the sum of happiness and reduce suffering, the decision to deliberately give birth to a Down baby, when you have the choice to abort it early in the pregnancy, might actually be immoral from the point of view of the child’s own welfare. I agree that that personal opinion is contentious and needs to be argued further, possibly to be withdrawn. In any case, you would probably be condemning yourself as a mother (or yourselves as a couple) to a lifetime of caring for an adult with the needs of a child. Your child would probably have a short life expectancy but, if she did outlive you, you would have the worry of who would care for her after you are gone. No wonder most people choose abortion when offered the choice. Having said that, the choice would be entirely yours and I would never dream of trying to impose my views on you or anyone else.”