Mince I don't know what the case is since you might be lying, so please listen: you cannot give a girl an abortion without her consent. It's wrong no matter what, but especially if the child isn't yours. Foul play is also very easy to detect by examining the placenta, and if this comes back to you, you're going to be looking at a LOT of prison time. I know this isn't a pleasant topic, but it needed to be said.
If she is your girlfriend, really your best option is to put the neonate up for adoption. Having an abortion can cause more problems with conception later on, so more babies later might not be an option. If she does give birth you can keep the placenta even if you're adopting the child out. If it's an unusual placenta they'll ask to keep it for study, but if the child was born healthy you can deny them and keep the spoils for your self.