Secondly do you have a retroverted uterus? If you have a retroverted uterus it makes the baby appear smaller (which Drs equate to younger) than it actually is on a abdominal u/s, but as pg progresses the uterus moves forward and the baby appears to 'catch up'.
I did not know that.

Tilygirl, really hope this is what has happened and that'll it'll all be ok for you