Sonographers are expert in making measurements in 2D slices and they know how to acquire keepsake 3D/4D volumes and export them as videos. However, they don’t analyze 3D volumes, so often 3D information is not saved, only 2D frames, and maybe a few short image sequences. Many ultrasound systems also cannot export the 3D volume in standard format.
The best you can do is to ask the sonographer to give you all data that they acquire, not just the video capture and hope that you can find 3D information in it. If you want to know for sure that you will have a 3D volume then contact the imaging center in advance and ask if they offer 3D printing from fetal ultrasound, if they do then it means that there are people there who know how to acquire and properly export the 3D data.