I agree with Ems mum - the fact that you are voicing doubts could be significant - at least you have a few days to mull it over and decide.
Of course, many of us on here had to make decisions regarding "surprise" pregnancies and have gone ahead with the preganancy - and although it's not often you hear people complaining that they went ahead with a pregnancy, sometimes they do!
I think also that many of us have had an abortion at some stage and thought it was the right thing to do for our circumstances at that time - and some may even regret that decision now.
In the end you need to follow your own head AND your heart - if you are having doubts, I would take that as a sign that maybe you DO want this baby, despite its unplanned nature - but I'm also pro-choice, and don't want to unduly influence you - you, and only you need to choose the right choice for you and your family - and if that means termination, then so be it - just don't allow yourself to be swayed by the father, or well-meaning friends, because you are the one that must live with your decision in the end - right or wrong - baby or no baby.
Whatever I have posted is my PERSONAL OPINION, not official single mother forum or legal advice. Just fellow-single-mummy ramblings - take it all with a bucket of salt!