Because this an incredibly complicated subject, I am going to defer to someone who has written much more in-depth about this than I can. This is my favourite quote:
"Screening and selection say nothing about the inherent worth of people with Down syndrome. They say everything about the elevation of the capacity for economic achievement above other human traits. My children are fascinating, demanding, delightful, present, annoying, dependent, loving, cuddly, different, unpredictable and completely human, just like other children. They are not a mistake, a burden or a reflection of my “personal choice,” but an integral part of society."