I thought the not being 'allowed' thing was more about the fear of an increase in the risk of stillbirth?
I thought that was about going past 42, or the magical 40+10?

Her first was born at 40 + 10, so the 40 week limit is very much about not wanting her to have too big a baby. Firstborn was 3.7kg, after a lot of scans and hoopla and carry on about how big it would be in the weeks leading up to the birth.