I heard that the most accurate dating to go by is the first u/s you ever had. ie: I had mine @ 9 weeks and it showed my due date to be 13th June , even though Ive had heaps of ultrasounds since then , I stay with the 13th June because the bigger the baby the harder it is to pinpoint the date
Good luck with everything , and remember its your body your baby and YOU make the decisions.. keep us updated!