The u/s is more accurate than the date of your last period.
this is because you do not know exactly when in your cycle you ovulate, you could be an early or late ovulator.
Also you could have implanted early.
The u/s shows the gestational size of the baby. This is based on how long it has been developing.
Most women implant around 10 days after conception. This can occur as early as 6 days after conception, so if you implanted early this could affect the gestational age of the baby and there by based on the "averages" appear as though your conception date was the 9th.
Go with the u/s
