it depends on who is doing the u/s too, this is what I've seen and experienced.
2 days behind is fine.

hang in there because, girl, you'll be worrying from here on out

It's just what we do. lol
You know how they talk about babies having growth spurts? they do in utero to.
I have to telly ou that to me, the 2ww was
nothing compared to the stage you are in. the worries about every single twinge, regular twinges that you'd never notice if you weren't pg. trust me. It's being on this side of pg that I have noticed that.
Hang in there and don't forget to breathe

hugs
keep us posted
jm

me:43
FSH 26
DS: born by c-sec Apr15'03, 9lbs5oz 41wks gest. (after 4 years of ttc, starting in 1998)
DD born by c-sec Oct 13 2007, 8lbs13oz 39wk gest. (after just under 3 years of ttc)
~~DS was conceived naturally the cycle following a cancelled DE IVF, using my good friend's eggs. She was on the verge of hyperstimming.
~~DD's nat conception I attribute to using OPKs like a crazy nut, eating grapefruit daily and using preseed. also 5 cycles of TCM ending 2 cycles before that lucky cycle.
~~~~~~~~~~~~~~~~~~~~~~