First of all, Wildcowboy is right about the way the pregnancy calender is figured out. If there hasn't been any overlapping of relations *cough cough* you're fine. 
That said, I would go ahead and have the blood test. If you and she decide to raise the baby together, it will be yours in the most important sense - because a parent is someone who is there and taking part. That's a LOT more important than biology.
And it's smart to know who the biological father is in case there are any medical conditions to know about, or in case of something like the need down the road for an organ transplant. Those things are traumatic enough in themselves; you don't want to have to deal with paternity issues at the same time.
There's of course also the issue of the biological father or grandparents maybe wanting to be a part of the child's life. It can seem overwhelming at first, but it really can work out, and when it does, it can be a great boon to the child to have a whole gang of adult resources in his/her life. And it can give the parents a little help now and then, which BELIEVE ME, can be very nice. And besides, it gives you more information for how you want to plan your life at this point.
Find out now, find out together how you want to deal with it with any involved parties, and then go on from there.
And good luck to all, whatever you decide.
That said, I would go ahead and have the blood test. If you and she decide to raise the baby together, it will be yours in the most important sense - because a parent is someone who is there and taking part. That's a LOT more important than biology.
And it's smart to know who the biological father is in case there are any medical conditions to know about, or in case of something like the need down the road for an organ transplant. Those things are traumatic enough in themselves; you don't want to have to deal with paternity issues at the same time.
There's of course also the issue of the biological father or grandparents maybe wanting to be a part of the child's life. It can seem overwhelming at first, but it really can work out, and when it does, it can be a great boon to the child to have a whole gang of adult resources in his/her life. And it can give the parents a little help now and then, which BELIEVE ME, can be very nice. And besides, it gives you more information for how you want to plan your life at this point.
Find out now, find out together how you want to deal with it with any involved parties, and then go on from there.
And good luck to all, whatever you decide.