Hey Honey,
I went in for my checkup when I was 36 weeks with my first and she was measuring 32- 33 weeks. I asked for one of the Drs to be bought up to measure and have a look yet they all ended up measuring from a different point on my pubic bone up to the top of my belly. My midwife was concerned and talking about extra ultrasounds and the like yet I chose not to have any intervention. Nothing ended up being wrong, she was born a healthy 3.45kgs and 52cm long. These days you may find a lot of people find the fundal height measurement to be very inaccurate, especially during the last weeks.
Hope you are feeling less anxious. I know I didn't feel the best but If the ultrasound makes you feel better than get it done.
P.s I got bigger in the last 2 weeks!