I worry too, I still havent felt him/ her move so I think you get over one worry and then you get another!! They take all the measurements again at your 20 week scan and do it all properly then, they would have told you if there was any concern that the baby was small. I think mine is a bit big from the 17week readings but they can never know exactly how far along you are, i dont know when I ovulated!
Good luck hun xxx