Yep, they are quite accurate. I did not know my LMP because I got pregnant right after a m/c. A sonogram was my only way of estimating my due date. I had a sono at 8 weeks, 12, and 20, and my due date has remained within 3-5 days.
very accurate. mine was to a T.
I agree with amberd..in the first trimester, it is very accurate!
Ultrasounds in early pregnancy are very accurate at dating. Good luck!