Ultrasound imaging uses the reflections of ultrasound vibrational waves between different materials to map out where the boundaries are. A transducer produces the 3.5 to 7 MHz vibrational waves which, for an image of a fetus, are directed into the mother's abdomen. The waves are reflected and refracted as they pass through different kinds of tissues, just as light is when it passes through different materials, such as air and water. The reflected waves are collected by the transducer. The larger the difference in density between the materials, the stronger the reflected signal will be. The ultrasound waves are scanned across the mother's abdomen to image the fetus inside.
Is this manner of creating images from reflected ultrasound waves similar to any other reflection imaging techniques? How is it similar, and how does it differ? With which techniques do you see a cross-section of the target, or just the surface?

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http://home.hkstar.com/~joewoo/joewoo2.html
for information about many aspects of using ultrasound in obstetrics.

I can answer this question about Ultra Sound from personal experience! Two years ago, I had to have my gall bladder removed because I had gall stones (that's when your body produces hard "stones" out of your bile, or digestive enzymes from your liver, and blocks your digestive enzymes from digesting your food!). They found out that I had these gall stones by using Ultra Sound technology.

The way the medical doctors use Ultra Sound is to create high frequency (higher than our hearing range) sound vibrations. These waves enter your body ( but don't harm you), and get reflected off different types of tissue, and then are returned back to the sending instrument. The Ultra Sound instrument organizes these sound waves based on 1) how long it took for the waves to return after sending them out, and 2) what frequency the waves were when they returned ( They will be less energetic when they
bounce back to the receiver...do you know why?). From all this information, the computer inside the Ultra Sound instrument can form an image of your insides!

I couldn't see the gallstones from my Ultra Sound video either, so I know what you mean when you say that you can't see anything on those videos. Fortunately, the nurses and doctors who take care of us have had years of training in looking at these pictures. (It is kinda like those puzzle pictures that you have to focus your eyes on in a certain way in order to see the hidden picture within it, right? Once you know the trick to seeing the image, it is much easier to do next time.)