Your own, unique cycle dates are way more accurate than mathematical averages - who is average and how can a computer tell us! The fact that only 3-5% of babies come on their due dates is testament to this. Anyway, no matter what, it's your body and your decision to make. He may try and pull rank, but he can't force you, it's against the law. Follow your instincts, trust your body and your baby. Also the later the scan the less accurate it will be, genetics takes over. Takes a big baby and a small baby to make an average baby - so forcing them all to be average just doesn't gel well with these little beings.