All sorts of arguments are used to explain the dreadful lack in quality of life that the child will experience in order to get parents to make the "right" decision. It seems to work, because, according to the most common statistic I've seen on the internet, over 90% of babies with Down's Syndrome in America are aborted. That works out as 9 out of every ten babies. And, in 2009, a report in the Daily Telegraph estimated that three babies with Down's were aborted every day.
People are attempting to redress the balance. Francis Phillips, who has a child with Down's, has posted an excellent article in the Catholic Herald on the topic, and she highlighted the following video...
Some people suggest that by aborting the babies with the abnormality the disorder will be eradicated. However, it doesn't work like that. The chromosomal abnormality is caused by the faulty division of chromosomes in the gametes of the mother - two copies of chromosome 21 are carried in an egg rather than one, and when the egg is fertilised, the sperm adds another copy. The faulty division of chromosomes is impossible to eradicate because of its random nature (it's a mutation) - though the frequency of this fault is known to increase as a woman gets older.
So the destruction of foetuses with Down's Syndrome is akin to killing an individual who suffered a major disability through an accident or illness in later life...
Oh, sorry, I forgot. That sort of thing is being done already.