Can the ultrasound be off by 8 weeks in the 2nd trimester?i mean if all ultrasounds match with the dates the preg. can actually be further along?

Very unlikely: ultrasound should be reliable within 2 weeks at this point, 8 weeks is very improbable

One week max: I think 8 weeks off between dates and ultrasound is unlikely. The maximum I've read in studies so far has been 6 days off with accurate dates. Are you sure when your last menstrual period began?

Due date & Ultrasoun: Earlier ultrasounds are more accurate for predicting the due date, still it could be off by 1 & half wk, between 18-28 wks the margin of error increases to 2 wks and after that up to 3 wks but not more than that. That's why doctors will usually use the dates and measurements from the first ultrasound of the pregnancy as a reference.