It can be off by a few days
It takes two weeks before the egg actually plants. So the first two weeks of pregnancy your not nessicarly pregnant yet
mine changed...it was feb 19 based on my lmp but then at the ultrasound they measured my baby and baby measured bigger so they changed my due date to feb 13. but i know that cant be. i went a month without sex and counting backwards....unless it takes sperm 2 weeks to fertilize an egg i know its gonna change again.
I'm sumwat in the same position my Dr goes by lmp with no ultra sound and says sep24th...but I had to have an emergency ultrasound at the hospital in which that lady said Oct 2nd...so I'm not sure either just going by the later date...he will be here soon either way :) hope u gt ur answer tho