LMP is last monthly period.
Hope your OB can shed a little light for you. All the best!

ETA: I know you said they are a non-event (lucky girl! LOL!) but can you look through your diary at things that you were doing (e.g. going out somewhere like to a friends' place) and think, was I riding the crimson wave then?
It's a tricky one because if your period was irregular anyway that can make the due date different...