I have a girlfriend and we've been going out for 8 months. Although we have been going out for these past months, something still bothers me as much as those men out there who have the same issues - she's someone else's mistress.
Whatever you do, make sure the first thing you do when the child is born is find out if it's yours. Do not sign anything until those tests come back. Ask a lawyer what to do, because this could turn into an expensive monetary problem long after the relationship comes to an end.