I'm not really sure!! Maybe someone else can help with that question - I just kow that in majority of cases they don't ultrasound for size unless there may be a problem?? I was overdue and even then they didn't scan for size... maybe different hosptials are different too. I do know that lots of ladies have said thye had a scan with an estimate of size and it turns out to be completely inaccurate!
If I was was you I would write everything down and check at your next appointment hun! yOu have every right to ask and get answers if you are worried!
Good luck!