second due date was September 2 When did I conceive?

You really can not calculate when you conceived since you had sex December 7 which falls with the august due date and the September due date. If I were to give an estimated guess I would think the September due date is more accurate but thats just a guess.

*First Day of Last Menstrual Period: November 21, 2009
Probable Date of Ovulation: December 5, 2009
Possible Dates of Conception: December 1 to December 9, 2009
Due Date: August 28, 2010 (40 weeks)

*First Day of Last Menstrual Period: November 26, 2009
Probable Date of Ovulation: December 10, 2009
Possible Dates of Conception: December 6 to December 14, 2009
Due Date: September 2, 2010 (40 weeks)