The margin of error gets larger the bigger the baby gets. I remember when DS was measuring around 10 ounces, the margin of error was like an ounce. When DS was measuring 5 lbs, the margin of error was a pound or maybe 2. (The tech always told me the margin of error that the computer calculated along with the pounds and ounces.) That being said, with both of my kids so far, the ultrasounds were really close. Olivia was estimated to be 12 oz (and that tech was being overly generous I think under the circumstances), she was 10.5, Lucas was measuring about 5 lbs (I think) at 34 weeks, he weighed about what we expected...7 lbs 4 oz at 36+2.
Me (29) DH (30)
#1-Olivia Caetlyn-9-28-09-9-28-09, 23+2 wks, emergency classic c-section, class I HELLP, IUGR
#2- Lucas Oliver (rainbow baby)- April 2011, 36+2 wks, HELLP and pre-e free! (lovenox and LDA pregnancy)
#3-Matthew, late October 2012...mostly normal, 37 wks, (lovenox and LDA again)
My blog: http://www.butterflies-and-rainbows.blogspot.com/