Hi Heather
There is no way that your OB or anyone else can accurately measure the size of your pelvis. When they measure the size of your pelvis you are flat on your back. In this position your pelvis is at its smallest. You are also laying still, this is something that you would not be doing when you are in labour. Moving around alters the size and shape of your pelvis making it easier for your baby to pass through. I would suggest that you do some more reading on this prior to making your final decision.