Some times they never tell because 80% of The they are wrong..my doctor doesn't, but Thr tech told me what the ultrasound machine measured.
I'm not having anymore scans
Just know the ultrasounds aren't always accurate. With my daughter, they said she was measuring 9 lbs.... when she was born (the same night as the ultrasound) she was 5 lbs 13 oz.
you can ask your dr. to tell you when you go for a ultrasound