How is it possible for ultrasound to determine conception date may 9th if i didn't have sex until may 11th? or might i be pregnant from sex on 4/30?

Margin of error: There is a margin of error of ultrasound exams that is about 5-7 days in the early first trimester. However, we also use the date of the last menses to calculate gestational age. In actuality, ovulations occurs about two weeks later. This is an antiquated method but has been around for a long time. Since it is only off by two days, it is within the margin of error.

Not that accurate : Ultrasound does not give an exact date of conception. There is always a built in error of a couple of days. Even in pregnancy measurements the longer the pregnancy goes the increase in the error that exists. So saying conception occurred on this day also involves an error of several days