You definitely are within your right to TRY and convince her to get an abortion. You obviously can force her, but you can attempt.
However, if she wont, its on you to man up. I mean you were man enough to fuck without a rubber. Supposed B/C or not, thats the risk you take. The OP may not be the one 'to blame', but he is DEFINITELY RESPONSIBLE! You knew the risks, and this was one of them.
Sorry bro but even if she was on b/c sometimes it doesn't work anyway, that shits only 99.9% effective. You could be that .1% regardless, so to say that the OP is free of responsibility b/c she "claimed she was on birth control" is such a load of shit that its unbelievable. So when me and my ex used to fuck when she was on B/C, its not my problem if she gets pregnant? Wrong, it most definitely is my problem.
Shit, even if she DID lie about being on b/c just to trick you, my point is still valid: this could've happened if she were on birth control. It is your responsibility fully, regardless of what she said, because you choose to dip without, as Lysis said. Now, her lying about this does in fact make her a person of questionable morals (shit, i'll say it, shes a bad person if she did this...), but that doesn't make you less responsible. Just like someone whos gf gets preggo while on B/c still has to deal with it, and its still their fault. If you really dont want a kid use a condom, birth control or not...
I took the chances with those odds, so does anyone who has sex without a condom, b/c or not. It is your responsibility man and your kid to take care of if she decides to keep it. No matter how you try and split things thats the ultimate answer.