Youre kind of screwed. She will see this as you lying to her from the get-go. And the longer you wait, the worst its going to get. If you are going to tell her, tell her asap.
On the other hand, if you tell her a year from now, it may not matter to her, if she really likes you--she may look passed the fact that you lied. On the contrary, if she really wants kid, she may see you as too old to have kids with and leave you, thus you just wasted a whole year.
So moral of the story: tell her asap.