If ultrasound shows me at 5 weeks pregnant today when did i conceive. unknown lmp?

3 weeks ago: The pregnancy is dated from first day of your LMP, whether from your memory (surprisingly, this is often the most accurate dating method), or calculated from a first trimester ultrasound. Conception is assumed to have occurred 2 weeks after the first day of the LMP. So date of conception was most likely 3 weeks ago.

Guesstimate only: The ultrasound is only accurate plus or minus 2 weeks so it is difficult to "backtrack" to a accurate LMP and date of conception.