The ultrasound say i conceived on 9/16/12, but i wasn't with anyone at that time. however i was with someone on 9/23/12. how accurate is a ultrasound?

Sooner is better: We measure embryos in mm's and that correlates fairly accurately with age in days/weeks. You can imagine that after 3 months, measuring a baby in mm's may not make much sense anymore. In the 2nd and 3rd trimesters, we're measuring in cm's -- then it's up to genetics, nutrition, etc. Although I am fp, i personally disagree with changing a due date by 1 week. Go with period and conception/sex dates!