I watched a number of these ultrasounds when my wife was pregnant. All of the actual diagnostic work was done on the traditional 2D video, even though the machine supported the fancy 3D stuff. At least for the machine they had, 3D had <10fps, while 2D has 60fps. Also, what the doctors are really interested in is measuring the length, width, and spacing of a variety of different bits. With the 2D, you just have to get things the proper orientation, freeze the screen, and then use the built-in "ruler" tool to get you an exact number.

So far as I can tell, the 3D business is all about eye candy. One clinic that we went to offered to sell you fancy videotape packages for an extra $200 or so. We stuck with the basic diagnostics, thanks.