Hey LMP means last menstual period - before your scan you will get a estimated due date based on this because many women don't know when they ovulated.
At your scan, like Sunbeam says they may change the date based on fetal growth but I'm not sure if that is accurate if it is just based on growth. My dates have yet to change, like the idea of the 9th September so if they tell me anything different at the 20 week scan I will ignore them hahaha
Xx