My thoughts on the subject are complex, and I've put a lot of thought into situations like this.
While being pro-choice (despite my generally rightward leanings) I believe it should actually be illegal to have a baby that can be determined to have severe birth defects or severe retardation while it's forming. Such a child is going to be a burden to society, and while I can understand parental attachment I think this is very similar to how we legally regulate inbreeding.
In previous years when we knew more about mental retardation it was a touchier subject. People would point out that guys like Einstein and Lee Iacoca <SP, I can't spell it> would
be considered retarded as an excuse for not killing or aborting defective babies. At that time we couldn't determine the extent of the problems as reliably, nor did we know as well what they would develop into.
Preventing things like this is one of the big reasons why I am such a big supporter of genetic engineering, human eugenics, and a lot of things that a lot of people are very alarmist about. I want to see such technologies developed and distributed to the point where such problems can be identified and solved easily and affordably. It also leads into other social issues, right now people argue that if they aren't religious or morally opposed that they should be able to marry their sister or whatever. I don't much care for the idea, but with such tech social/moral questions like that go away and you can pretty much tell them "knock yourself out if you both really want to".
I'll also say that things like any kind of "mercy killing" become touchy when humans and human emotions are involved. Rational thought oftentimes goes out the door. A lot of people will point to retarded relatives, children, or siblings who they love and will point out get by. However those people get by with a lot of public money and special assistance, not to mention the simple question as to what happens when the relatives caring for and assisting them now are gone, then they become an even bigger burden on society. What's more contributions by such people are frequently argued, but rarely go into the cost of the assistance that got them that far. Sure you might have some retarded guy who has worked at Mcdonalds, or a mail room for 10, 15, or even 20 years... however over that time period how much was invested by the state through education, employment assistance, special training, assisted medical care (drugs, constant monitoring of the condition, etc...) and the like. Most of those programs doubtlessly involved contribution of taxpayer money, unless filthy rich it's doubtful the family was able to shoulder the burden of a retarded child/sibling unassisted.
I am not anti-charity, I mean there are plenty of people who develop medical conditions later (which will also hopefully be corrected), injure themselves, or whatever else that put them in a similar league. Heck, I'm retired on social security myself. Dealing with all these things are fine, and I agree with making it a moral responsibility worthy of using public money. However I do not think intentionally dumping retarded and badly damaged offspring into the pile is a good idea, that's asking for trouble. As our recession shows we need to be wiser with how we use our society's resources, no one issue like this is responsible, but every little bit you remove winds up helping to keep the economy strong. Choices are never easy but you DO have to set priorities and I think maintaining badly damaged babies into badly damaged lives is pretty low on that list of priorities.
Apologies to whom this offends, this is simply my opinion. I'll also say that part of my thought processes are that while relatively intelligence I apparently suffered brain damage after I was born, I had some kind of condition that had my head closing up and a plate was placed inside and then removed. I have these giant dents on the top of my head and have dealt with emotional problems through my entire life. It's all well documented, but I'm still not entirely sure if I was basically born broken, or if the doctors messed up. The bottom line is that even while relatively intelligent, I don't think I'm a boon to society, and I wouldn't want to put anyone through the pain and problems I've had to endure.