Of course its fact because it came from a man with a medical degree Seriously though, it might be 'fact' that they use an u/s machine to estimate dates, but from a biological point of view, it means nothing. Size means nothing in relation to due dates so the u/s is really quite pointless IMO. This is why they say a 'normal' pg is anything between 37 to 42 weeks and I reckon that those women who have their bub induced at 42wks and out comes a baby still covered in vernix and a perfect placenta could probably go for a lot longer yet iwthout drama. Being pg and giving birth is the work of mother nature and she isn't exactly an exact science