Those scans are crap though, often they are done with you lying down, not under the influence of labour hormones. In labour all the ligaments loosen and baby's head has 4 separate bones which can mould and move... it is well known that scans are not advisable to judge the pelvic capacity, because lying on your back minimises pelvic space, closes it in... to make more pelvic space you can squat - up to 30% more space... urgh irks me that they still do it. There is a Cochrane Review for this even, saying they are not ideal to do to judge the pelvic capability!!!