I'm going to tell my newly pregnant girlfriend that she needs to consider my opinion on whether to keep the baby
How do you think she feels about the situation?
She may know that I am against her decision to keep it.
It sounds like you are in a tough situation. Talk to your girlfriend about how you feel about the issue. She should consider your feelings, but know in the end it's her final decision to make.
I think I'll still tell her that she needs to listen to me before we discuss anything.
It is right for her to give you a chance to say your piece. But at the end, it is her body, so she has the final decision. You need to be ok with whichever option she chooses.