What they said, plus, those measurements can be off. Just depends on how baby is sitting and probably how the individual doing the scan sees it. (For example my RE took three measurements and then picked which one she liked best which opens it up to personal judgment). My OB the last two pregnancies and my RE for this one all said they wont even bother changing the due date unless its 3-7 days off.
Meg, Momma to Joey (Feb 2010)