Couple questions:
1 Did you have sex prior to 8/10 ?
2.When was her last period?
Ultrasounds can always be off, but also, DR's date preganancy starting with last period, not conception date so it does get confusing.
If she is 14 weeks pregnant, then it's about 12 weeks since she actually conceived (give or take a week). Confusing I know, but that is how they count. Most women ovulate approxiamately 2 weeks after their period starts. So if you count backwards, that puts conception date around the 3rd, which is close. This is not an exact science. A few days off on dates or a non 28 day cycle would explain the week's difference. The only way to know for certain is going to be a DNA test because it is certainly possible you are the father.