im confused as to why you need to know when her lmp was....you all found out shes 6 weeks pregnant on april 14th.
if her dude date is december 11th, 2009...her LMP would be March 7th..conception would have occured march 21th, 2009
her scan is likely a rough estimate....it is not exact nor is her due date...they likely have moved it slightly now...being that shes almost 19 weeks
If she had a scan on April 14th that revealed she was 6 weeks pregnant, conception would, in theory, have been around March 14th. Are you sure your last sexual activity was in February? Is that the issue?
If so, first of all, please understand that one of several things might explain it. 1. You could be wrong on your memory of when you last had sex. 2. The early scan was not exactly on the 14th of April. 3. The doctor's estimate at that time (of 6 weeks pregnant) was incorrect. 4. She might have ovulated at a different time in her cycle than is the norm. If the baby is born well before December 11 and seems full term, I would wonder if one of the above was going on and gave you incorrect dates.
If a question of paternity is the issue, you certainly have the right to ask for a DNA test when the baby is born. Good luck!