LMP, or Conception date?

srd3

So, I was reading a post on the Dec 12 moms board (since I'm due beginning of Jan, I lurk on the Dec and Jan board) about how many months are we really pregnant, 9 or 10. And it got me thinking.. Do I count from my last period, or from the day we conceived (since we know the day and time down to the minute..lol)..due to IUI. Makes me wonder which date my Dr is using, I think I vaguely remember her saying the conception date. How are you ladies counting and does it make a difference? .