Ultra sound accuracy for a newbie

I have never had any children before,so I am a total newbie at this so forgive any seemingly common sense question in the area of pregnancy. I visited my ex girlfriend on July 8th 2011 and stayed for about a week. We hooked up obviously since I'm here and not on September 13 the doctor said she is 11 weeks and 4 days. I found out recently that she broke up with her bf in June and they were together the middle of June, "from what she says." When i confronted her about she said it doesn't matter when she hooked up with him because she says she had her period late June. However the 11 weeks and 4 days doesn't add up to me...any help would be appreciated.

[alaysha]

They count pregnancy from the first day of your last period. Meaning she concieved sometime around July 8th-15th.

[alaysha]

They count pregnancy from the first day of your last period. Meaning she concieved sometime around July 8th-15th.

[alaysha]

When I put in the dates it says her last period was approximately June 24th and she concieved around July 8th.