Last period december 8/16 was giving a due date september 14/17, a 12 week scan gave me a new due date of september 9/17. earliest conception date?

About estimates...: Dates are just estimates, and a few days does not make any difference in almost all cases. One can just get a calendar and count backwards 38 weeks from the estimated due date (38 weeks from 9/9/17 or 9/14/17) to see the estimated conception date. The exact conception date of a natural pregnancy is never known because nobody can tell when the sperm entered the egg.