lindsay
The usual way to figure out when you got pregnant is to calculate 14 days after the 1st day of your last period. It doesn't matter when your last period ended, some women's periods last a week some last only a few days. If the first day of your last period was Dec. 25, then you probably got pregnant around Jan 7 give or take a day or two. If you are figuring that Dec 25 was the last day of your period and say you started your period on the Dec 20th then you probably got pregnant around Jan 2. You probably did get pregnant on Jan 2nd if they say your due date is Oct 2.
Hope this helps, Good luck with you pregnancy!