If I had sex on 3/9/11 and I am now 11 weeks with a due date of dec.4 or 6 and the first day of my last period was 3/1/11... is this the day I conceived?

Yes, it is likely you conceived from the sex on 3/9/11. The due date is derived from the first day of your last period however, as it seldom is known exactly which day someone conceived.

You can only conceive when you ovulate... one day. However, sperm can survive for a few days, some for several days, in the days leading up to ovulation.

You may have had sex on the 9th, but you may have actually conceived anytime in the next several days after that.