Hi! I had sexual intercourse with my girlfriend last July 21, 2012 and on July 28, 2012 her menstraution came... second sexual intercourse with her was on August 26, 2012 knowing that it's safe but I found out that she was 11 weeks and 2 days pregnant on October 27, 2012... I just want to ask if it is possible that I am not the father considering that on her ultrasound result the conception date was on August 11, 2012? Was the ultrasound accurate? Please help... thank you

While it appears you are outside the window of error, ( the other man is almost outside also) Did she have sex with him other dates ?

But the ultrasound is a guess, a good guess but can be plus or minus about two weeks at times. So the date given could be plus or minus about 14 days.