They checked my fluid levels (and other things) at the last few ultrasounds I've had the last few weeks (i've had several ultrasounds lately). I saw her go to four different quadrants, use the cursor (or whatever) and clicked on two separate points, like at the top and bottom of the black pocket of fluid, which the computer gave her a measurement for. They added up the size from the four different quadrants to get an average. Each of the quadrant was a different pocket that looked like that black space you were mentioning.
I think they have to be at the right frequency (or whatever) to get to that black pocket, because I didn't readily see it until she did that particular measurement. I thought it was pretty cool, I wish I knew more about sonography.
