It could indeed be a big deal, if you said the last period was significantly (weeks) earlier than it was and your baby happened to be on the large side. Your doctor may well think you are near your due date and schedule an induction, and your baby may technically still be premature and have complications. I had an issue and did not know my LMP, because I was on the pill and had had irregular bleeding. It bothered me and I wanted my doctors to help me figure out which one was "real" because 3 weeks is a big difference when we're talking about growing a baby.
Honestly, though, conception date and due date mean nothing, if you don't let science interfere too much. Our bodies have been baking babies to perfection since the beginning of humanity (and all other mammals as well!). A date on the calendar doesn't mean nearly as much to your body as the hormonal changes and natural processes that lead to birth.
at 4:27 PM on Jul. 2, 2011