Quoting Amiyah Destiny's Mommy:" <blockquote><b>Quoting Dahling:</b>" The same thing happened to me--I got pregnant ... [snip!] ... everything by Friday. Started having sex Saturday and its been non stop , no pulling out nothing. I was 12 weeks pregnant."
Well, if you got a positive already, I'd assume that your pregnancy would be dated the same as if your last period was on Dec. 17th. Obviously I'm not a doctor though, so I can't tell you for sure. haha That's just the way it worked out in my case, and all the office people were using my m/c date as my LMP until I had an u/s to confirm my due date.