My doctor said if your charting, and know when you ovulated etc, to go by those dates or the first day of your last AF as both are more likely to be accurate than the ultrasound.

U/S dates can be out as they assume an average size for all babies at birth, and all babies just aren't the same size! Although earlier u/s date tend to be more accurate than later ones (i.e. a 7 weeks u/s is more accurate than a 30 week one)