Early U/S

How accurate are early ultrasounds and why do some say +/- 14 days after you EDD?

my earliest ultrasound with this pregnancy was at 8 weeks and i get a ultrasound done every single week and every week she measures to be due july 21st. So i'd say about 8 weeks....

i had an early u/s with this baby at 6 weeks and i didnt have that on mine, my due date stayed the same.

i got my first u/s at 5 weeks and all they saw was the sac and when they mes. that it gave me the due date of july 3rd them at 6w i got another and it said kuly 3rd and 7w it said july 3rd and then 11w july 3rd but and the same with my 19 weeks and 27 week but at 32 weeks i started mes. small so they checked me and he was a few days off and then at 37 week he was 3 weeks behind and now he is mes a week behid but they say its just cuz hes going to b short!

soi say that they are acurate from 5 week lol

the +/- thing is becuz a baby can come anywhere from 2 weeks earlier or later then that becuz the date is estimated

Also with my first daughter I was due Nov. 11th 2005 and guess what I had her Nov. 11th 2005 without geting induced or anything! That was Right on Target!!!! haha

I was just wondering because with my other kids none of my ultrasounds say that (theirs werewithin a few days of my EDD) but with this one I had my u/s at 10 weeks and they said I was acctually 11 weeks and some change +/- 12 days. So I was just curious.