The only reason why she has changed it is because of the first u/s at 6weeks that i had. it looked more like 3-4 weeks. so she is thinking i concieved later on in november.
The ultrasound would have shown if the baby was measuring too far along, or slowly developing.
I know you dont mean it in a bad way, but im pretty sure that if that were/is the case, they would inform me about it.
My fiancee also counted from when i was ovulating after the first u/s and thought that i most likely concieved around my ovulation day.
where as with doctors, they use the wheel and go by the last period. When you dont get pregnant on your period[yes it does happen sometimes but is rare] and that is an estimate for when you would be due.
When i go to my next OB appt. i will ask her why exactaly she moved the due date, and if the baby is developing well and such.
I remember my sister inlaw said that she always was a week ahead of her pregnancy when she would get ultrasounds done and measurements.