I'm going to apologize in advance because what I remember about this fic is rather vague, but here is what I do recall -
1) It was an AU that's pairing was Dean/Cas (I can't recall if it was already established or if the relationship built over the course of the story).
2) At one point or another, Dean and Lisa were in some type of a relationship and I believe that Lisa either left or skipped town without letting Dean know.
3) Lisa returns and it is revealed that she is pregnant and she lets on that it is Dean's child - from what I remember, Dean was pretty excited about this.
4) Over the course of the story they start getting things ready for a baby - home improvements, etc.
5) In the end, it is revealed that the baby isn't actually Dean's - either Lisa skips town or she lets them know. Dean's pretty broken up about this (and I think Cas might've been too). Also, I think Dean & Cas may adobt a child (but this could be just me mixing fics together).
Again, I realize this is pretty vague, but that basically the gist of the fic.
I don't think the fic was on AO3, I'm pretty sure it was on LJ (but that doesn't mean it hasn't since been exported over or deleted). I've been in and out of the SPN fandom, so I'm figuring this fic is at least 3+ years old.
Any help would be greatly appreciated :D.