I am so worried sick over this. I do not remember the exact first day of my last period, but I do believe it was aug. 15th. I am sure I had sex with my boyfriend on aug 21st. I was with someone else (not my boyfriend and not someone I want to have a kid with) on aug 31st. The ultrasound technician said that it shows that aug 15th is when I had my last period. She says it measures exact... I wanted to ask her if that means that I am really 6 weeks pregnant, or if she is still going by the LMP as opposed to the actual measurements of how pregnant I am. I forget how many cms she said I am, but the ultrasound picture says 8cm. Am I really 6 weeks pregnant, or am I 4 weeks? Is there any way to find out how many weeks exact I am so that I can count back and make an accurate time for who the father is? This is VERY important to me!

Yes. Estimates of conception is just that. A educated guess. It uses the date of your last cycle, and average size of fetus. At the very best, the most exact it can be will be give or take one week. It can even be off up to two weeks.

With sex within 10 days of each other. There is no way to know at this point who the father is. You will just have to wait, till the baby is born and do a DNA test.

Sorry, no one can tell you at this point

The tech was WRONG. Or you misheard her. Or she wanted to calm your nerves. She can't know when your LMP day was.
Let's say you wrote down Aug 15. You STILL don't know who the father is.
The ODDS are that the sperm that met the egg mid-cycle is from the father, but odds are just that - they aren't proof. If you have a typical cycle, and a typical ovulation, that would make the 8/31 sex more likely. BUT sperm can last up to a week before impregnating an egg (most likely would be about 3 days), so that helps the odds for the 8/21 sex. If the ultrasound shows a bigger embryo, then fine, take a deep breath and hope. But nothing will be proof, nothing, until after the baby is born and you can have DNA testing.

You might be thinking 8mm for baby's measurement. Early on, development is pretty consistent, so accuracy tends to be within several days either way. Ultrasound becomes less accurate at the end of pregnancy, where it can be off by weeks either way.If baby measured 8mm, you are most likely about 6+ weeks.As was said, complete certainty would require DNA testing.