Yup I agree with everyone else!!!! They are just "guessing" with all these measurements, and they are soo inaccurate. My last OB said that they take the dates off the first scan they do (like 6 or 8 weeks) as they are most accurate then, instead of 18-20wks when there is so many things to take into account - ie, bubs could just have long legs and throw off the whole dates!!!! ridiculous. And I think its true, we do create bubs size that we can handle.
I went to the GP the other day, and he felt my belly and was surprised by the size of what I'm carrying - and mentioned around 10 pounder! (Mind you, not what I wanted to hear when I'm having a VBAC) I went to my OB and she said, nope, probably around the same as DS - 8 pound something. She also said that we only make babies we can push out and it is very RARE to make a baby too big for us to handle.
My first OB said 12 pounder for DS - so a whole 4 pounds out - so not accurate in the way that they measure!!!!!