You need a fixed one for the line in the sand between prem and not prem for hospitals.

I first got an EDD of July 1st at my 8 week scan, then June 29th at the 12 week one. Noone has done any scans or came up with anything since then. The baby itself disagrees with both dates ...

Earlier dates are more accurate - as time goes on, that blob of cells becomes a little person that takes after the parents. Two huge parents make a huge baby that will measure way ahead on dates.

This whole measuring late in pregnancy seems stupid, every other story I hear is people being induced early or having a CS early because of a 'big' baby and it turns out to be either small or premature and ends up with all sorts of problems. Why not just let em cook until they're ready to come out on their own?