Timing...

So today I had my 4 week check up and my nurse said you’re 17 weeks today right and I replied no I’m 18 because when we had our 8 week ultrasound we were measuring a week further than our original conclusion based off my period....so when my doc came in she said you can’t push things along faster it doesn’t work that way and proceeded to tell me they are sticking to my original timing because they don’t go off of ultrasounds because they aren’t always accurate....so this whole time I’ve been going off being a week ahead of what my doctors office is saying...but like one of my first appointments she told me the first trimester ultrasound is the most accurate way to measure how far along we are...I also read that in one of my pregnancy books...so my question is did anyone else have this issue? Which would you follow...the ultrasound or your doctor??? I know it’s only a week difference, but it stills bugs me lol