“Size of fetus inconsistent”

Cierra

28 + 3 has anyone had issues with size of baby? Went to the doctor today and stomach was measured while laying on my back which I still don’t understand why they do that. But scheduled now for an ultrasound next appointment to see if she’s growing correctly. Baby’s heartbeat is always good and I know movement is great, I just guess she’s smaller than average for third trimester.