If I turn 12 weeks on oct 6 2011 what week was my child conceived

Unfortunately, it's not that easy to tell when you conceived in this manner, by knowing the first day of your last period I could help you a little more, but that's still a guessing game as we don't know exactly when the sperm met the egg.

In the end, if you are concerned about paternity, a DNA test will have to be done after the baby is born.

Yes sorry it does not work that way, first your number of weeks pregnant is only a estimated guess from various sources. It really does not matter since the date of birth is never by schedule and up to many factors.

If there is a paternity issue and it is within a week or two of the same time of having sex, there will be no way to tell till the child is born. You did not mention this, but this is normally the reason people ask that question.