Hi there
There is an actualy condition where there is too little fluid in the sac.
It was queried this was my case iwth DD as i didnt get any bigger in size from 33-39 weeks, although DD was growing correctly.

It is actually a specialised Ultrasound that took about 30 minutes where they have to measure all the fluid specifically.

They did this for me and gave me the all clear (dd was just doing tight yoga positions lol).

Did your sonographer do this?