Hi,
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If your due date is July 3rd, 2011, and you have regular menstrual cycles of 28-30 days, then your probable conception dates would lie between Oct 4th to Oct 14th. The exact confirmation of who the father of child is can only be made out by a Paternity test. If there was a period (menstrual bleeding) in between the intercourse with 2 different people, then the chances of pregnancy resulting with intercourse after your periods is high.
Take care.