I just know I'm going to be ranting this time tomorrow, I have a scan and I know full well that it will be about a week behind but they insist on going by lmp but because this little one was a bit of a surprise I can't tweak my lmp date to get the right match up.... sigh... never mind!
Every medical person I have come into contact who has asked for my LMP I have told them and then very quickly followed it up with 'but I don't ovulate until around day 20' which all of them have accepted and have adjusted dates accordingly.
If it was an oopsie then I'm assuming you're not completely certain of your exact date of ovulation but I think it's worth mentioning that you don't follow the 14 day rule to save them telling you you're measuring behind and being all negative etc