Even with 6-week cycles, the doctors insist on going with my LMP to calculate my EDD, even though the scan showed the baby was way too small for that EDD (well, OK, I was having a short cycle, but they're still a few days out by the scan). I think it's really stupid!

And conception can happen in a supermarket, it doesn't happen during sex (well it can, but not from that round of bding), so no-one knows the exact date of conception, but knowing the date of ovulation would be a lot more of a help because that narrows it down to a 24-hour gap for conception, far more reliable than the LMP!