the further along the pregnancy the less accurate
as we had no due dates for my daughter and my first U/S was at 12w3d they just guesstimated from there and when she was born they guessed she was 6 days early
having said that with my 3rd my dates were inconsistant with my LMP and my last 4 U/S he measured a week behind my LMP and when he was born they must have calucated he was 3 weeks early not 2 weeks
my LMP had him at June 27th 2011 but my first U/S was july 5th 2011 so i took my 2nd U/S at july 2nd (fear of going over due to a crappy pregnancy )
but as i said he was 3 weeks early so all dates were off lol