Ultrasound Accuracy: Determining Gestational Age & Ovulation

Question
My question is how accurate are ultrasounds in determining gestational age. I know that ultrasounds done in the first trimester are more accurate than at any other point.  My last period was March 24th and on my earliest ultrasound done on 5-8-04 it says the age was 6 weeks and 3 days, on 5-24-04 the age was 9 weeks 0 days, so what was the possible date of ovulation?

Answer
A sonogram taken at 5-10 weeks has an error of +or- 3 weeks. Actually, the best time to take a sonogram for dating purposes is between 12-16 weeks (from your last period). At that time the error is +or- 1 week. However, most obstetricians take a sonogram at 20 weeks (when the fundus is at the level of the umbilicus) and when the error is +or- two weeks. In any case, based on your early sonograms as well as the date of the first day of your last period, you probably ovulated on April 6 and could have conceived if you had sex on April 5,6,7, or 8. The fetus appears to be growing adequately.