Some say that 12wk fetal ultrasound measurements can be off by 2wks, but some say only +\-5 days. which is it?

5 days: Ultrasound for predicting due date (fetal age), at that time is accurate, and if done well, the margin of error would be small. Even if not done trans-vaginally, 7 days would be the largest margin of error.

Fetal Ultrasound Err: The usual figure cited is +|- two weeks for us performed after 12 weeks: http://www.Newbirthcompany.Com/estimated-due-dates/. There are better measurements (such as fetal crown-rump length) that are more accurate as the weeks go by. Don't worry - your obstetrician will be able to "nail down" a more accurate due date in the weeks ahead!

Earliest is best: The earliest ultrasound has the least range of variability. The danger is to pick a 15 week length for estimated gestational age when it is earlier than that at 12 weeks. That is the road to missing intrauterine growth retardation of many etiologies, because the 12 week measurement has far less variability than the 15 week measurement, and is therefore more accurate. 12 weeks +/-m 5 days.