1) since you have got your girl friend pregnant you should marry her
2) if you refuse to marry her she may file case of rape against you under section 376 of IPC
3) it would be her case that you had sex with her under promise of marriage and after making her pregnant are refusing to marry her
4) in addition she may file case under section 420 of IPC for cheating
5) your defence should be that no promise for marriage was made . she willingly consented to have sex . hence no rape or cheating case is made out