my official due date was 4 days before the real one (I temped and we only DTD at the time of ovulation) and it really annoyed me. he measured 5 days ahead at 12 weeks, but they stuck with the EDD based on my LMP, even though I never have had a CD14 ovulation, it's consistently day 18 and a 32 day cycle. towards the end I was very very anxious because they wanted to induce at 40+12, and I was desperate to avoid induction, and very conscious that 40+12 was really only 40+8.
I went into labour naturally on 40+11 (40+7 real date) and he was born the next day. so pretty overdue. I had refused induction after agreeing to go in on 40+13 (40+9), I actually had an appointment arranged with the consultant for the day after he eventually arrived to discuss my options.
my first son's EDD was adjusted 4 days backwards at the 12 week scan, which agreed with the due date I had calculated based on ovulation. however he measured exactly to those dates. my second son being bigger at 12 weeks made such a difference to the dating, he ended up being a lot bigger than my first son (9lb 9oz vs 8lb 1oz).