If you have a 12 wk u/s and it reads 12 wks+4days, and if there is an 11 day range of error, wouldn't there be a different measurement of the fetus?

No: A 12 week ultrasound measuring 12w and 4 days is good. They probably did a crown rump length. I could be wrong but that measurement does not have an 11 day range? In fact, that measurement i'd so precise we will change a due date based on a crown rump length to the ultrasound due date if there is more than a 4 or 5 day difference between that date and the date from your last period; good luck.

Pregnancy Dating: For the situation you describe the infrasound due date may be the better estimate. At most centers a first trimester ultrasound is accurate to 7 says. Since there is a greater than a 7 at discrepancy from your menstrual period dating, the ultrasound date is used.