I had two due dates when did i concieve?

So i found out im pregnant... I had two due dates and my first due date is august 07, 2012.. nd the second is august 13,2012..I did have sex on november 5, 6, 12,17 without condoms and when i had sex on the 20 it was with a condom so does that mean i was already pregnant on the 20 ? Could i gotten pregnant on the 20 th even with a condom... The reason why i have two due dates is when i got sick around december 12, 2011 i had went to the hospital and they did my first ultra sound and i was at 5 weeks and 4 days pregnant but when i went to the OB she had gave me the second due date at 08/13/12... My first day of my last period was november 1,2011 nd the last day was november 4, 2012. So i really think i got pregnant befor november 20th... And now when i go in for my ultrasounds im always measuring 1 week nd 3 days bigger so could that mean i did get pregnant befor the 20th of november?