It's funny how they won't budge isn't it. My GP estimated the baby was a week older than the hospital dating scan by his scan and a doctor in New York estimated that same age when scanned at 23 wks, and I have my temperature charts showing that there is no way conception could have been when it would have needed to be yet the hospital won't change due dates.
At least it means I'm more likely to be medically induced which will be good.