my 20 week scan had a 10 day margin of error allowance. I think they commonly use the BPD and AC measurements to calculate weight, so the head size and abdominal size so I think you are right she is probably just short, but sounds like she is filling out well. I hope she can stay in long enough, safely to get a bit bigger. I know every day must be agonising at the moment. That moment when she is safely in your arms must feel forever away. Thinking of you.