It would be around 8 weeks before the ultrasound date, since you are not actually pregnant for the first two weeks. Pregnancies are dated from your last period. So period on day one, conceive around 14 days after that and therefore '2 weeks pregnant' at that date of conception.
I think its around the 4th October - this is also my due date from first ultrasound at 9 weeks, and i think they date back from last period which is 20th Sept, 2011 - and if you check with the due date calculators online they give an estimated date of conception of around the 4th October
2012 was a leap year, so: from Sept 28 2011 to Sept 28 2012 is 366 days; and from Sept 28 2012 to Sept 28 2013 is 365 days; a total of 731 days. 7 days in a week, so: 731 days ÷ 7 days/week = 1043/7 weeks ie just under 1041/2 weeks.