Well, I was just thinking, maybe you could consider adoption. There are soooo sooo many children in the world who need a loving family.
And since you already have two of your own... do you really want or need to have another of "your own"?
My uncle and aunt adopted 4 (I know!) children and I couldn't imagine a more beautiful or happy family. And I shudder to think of the life those poor kids would have had to live had they not been adopted.
Anyways, it is a very personal desision, but I just wanted to throw that idea out there as it seems like it would satisfy your emotional needs of a third child without the intellectual guilt of knowing that your child may be sick. As well, adopting will save you from having to worry about your own health through the pregnancy process.