As far as ultrasounds go (because ALL scans have a margin of error) is your 5-6 week scan. This is because at this point all babies develop at the same rate without genes taking over.

BUT your most accurate EDD is your own, based on your LMP. Even then, LMP calculators base the EDD on a 28 day cycle and 14 day ovulation so everything is far from accurate really! Thats why it's called an estimated due date. Just trust that your baby will come when he is good and ready. Oh and for date's sake, take the one which gives you more time and less pressure for an induction.

Personally I think there is far to much emphasis on EDD's and we'd be better without them! :P