What do you think is going to happen when you assume you're fertile, and have unprotected sex? A potted plant will fall out of you? No!
Edit: It's clear that some people know exactly what I'm talking about, some don't and some people need to dissect and pick apart what I am saying, to be argumentative. I'm talking about those couples, who stop all forms of birth control, become pregnant, and tell everyone they weren't trying. Also I do not subscribe to the "we're not preventing/but not trying" or "whatever happens, happens" logic. To me, that's trying. YOU KNOW what is going to happen, if you assume you are fertile, and it is not going to be a potted plant!
And if you are having sex with no form of birth control, but are under the assumption that one of you is not entirely fertile, that is not trying. Best wishes to those of you with that experience.