if your due date is october 4, 2012 - the person you had sex with on December 31 does not add up to being the father if your cycle is around 28 - 30 days. Which is most common. A due date of Oct. 4 would mean that you started your period 2 days before you had the unprotected sex. If you had sex anywhere in between january 8 and january 16, that person would most likely be the father.
First Day of Last Menstrual Period: December 29, 2011
Probable Date of Ovulation: January 12, 2012
Possible Dates of Conception: January 8 to January 16, 2012
Due Date: October 4, 2012 (40 weeks)