I had an interesting conversation with a friend of mine about this question last year. The desire, or urge, to have children for her is a constant thing. Like, she has always wanted kids (she has two now) and the desire didn't go away after she had the first one. She definitely feels like her body is telling her to do so.
Like most people on here (probably) I am missing the desire to have children for sure. Every way I look at it, the CF lifestyle makes more sense to me and is more enjoyable. However, there have been a few times when I've felt like having a child was something I might want to do. I agree with Kat1980 that this urge (which I felt around age 35-36) had more to do with the fact that I know biologically I only have a couple of years left. However like jeannie70, I was able to be logical about it, think it over and be absolutely sure I wanted to remain CF. It's like you know it's an option (and like other people have said, there's a whole lot of pressure with people telling you that you should have kids) but it doesn't totally take over your life. So that's my experience with the "biological clock." The urge to procreate may be a biological thing, but birth control has changed the outcome of sex so it doesn't always end with reproduction. We now have the ability to choose the outcome.