Mild retardation I could take. I would freely have a kid with downs syndrome as they are some of the sweetest people in the world. Something severe like what you are suggesting though is not something I could do. Such a child would never have the chance to grow up or become independent. They would have no achievements and no accomplishments. They would essentially be a trapped prison of flesh that required constant nurture that I know I would not be able to provide in the long term.