Take it to court, the ***** lied to you.
EDIT: By the way, I love the total lack of sympathy in this thread. Talk about a double-standard, huh? If a man told a woman that he's had a vasectomy, and they sleep together and she ends up pregnant, everybody would scream bloody murder and demand his head on a pike, but switch the genders and it's still the man's fault, somehow.
How about we all stop this jealousy-fueled "everything that goes wrong in a sexual situation is your fault because you have a penis" bullshit, and stop overlooking the fact that everybody in this situation should be held equally accountable. Actually, scratch that, considering OP was acting under dishonest pretenses presented by the other party, I'd say that this whole thing rests hugely on the female, and OP's life shouldn't be ruined because of one cock-trapping girl.
I understand that everybody that says "well, you're fucked, OP, that's what sex gets you" is probably someone who holds the concept of chivalry very highly, but you need to understand that chivalry is a two-way street. It's safe to say that if a woman tries to trap you into raising her child based on deliberate actions and blatant lies, she's not an example of "the fairer sex", and should not be regarded, nor protected, as such.
tl;dr stop giving OP shit because he's the male in the equation.