I am 13 weeks pregnant. My doctore says I am due September 3 2012 but when did I get pregnant

If your due date is 9/19 then it is possible that you got pregnant on or around December 20. However, if you are concerned about paternity a DNA test will have to be done as gestation is only an educated guess.

My due date is September 3 2012

Yes, basically you got pregnant most likely during Dec. there is no way to know an exact date, give or take a week ( even two weeks) is about as good as it gets.