baby measuring different week

Natasha
So the first day of my last period was 8/30 which would put me at 8 weeks today (Tuesday). I had my first ultrasound and the baby is measuring 6w4d. Heart rate was 138. UT said uterus is high so I have to go back in 2 weeks to get a better picture to make sure the first ultrasound is correct. Anyone else have this happen to them?