My personal take on finding all this stuff: When I started, I didn't have much of an opinion either way. I had read a couple natural pregnancy books that said don't do it if it isn't indicated, but no details. And the standard medical line is, it shouldn't be routine, but it isn't bad. So, based on this limited knowledge, I just wanted to know more about it.
1) It took a long time, I've had it for over a month, and have been looking for more info. That it was so hard to find info led me to a couple considerations:
a) perhaps there isn't anything to say about u/s
b) perhaps nobody bothers looking at it because it's accepted as safe
c) however, somebody better damn well be looking at it because they keep changing the technology all the time without any guidelines or further testing.
2) Doppler and u/s are different (you may say duh) in that one is looking for sound and the other images. u/s machines have built in dopplers, but they aren't usually turned on the whole time. when you hear the heartbeat via u/s, that's the doppler. It was even harder to find information about doppler than u/s. I found it inconclusive that doppler was more harmful than u/s simply because I couldn't find enough info or anything specific about the ranges and specifics about the type of sound waves and their effects after bouncing off of blood vessels as opposed to the sound waves of u/s bouncing off of skin, myelin sheaths, etc.
3) since babies heads are rounded, and there's a lot of brain development going on, the u/s waves can cause more harm from the energy created by the sound waves (not from the level of the machine itself, but once it bangs around inside the head) that can cause damage to myelin, which is the stuff around the spinal cord, neurotransmitters and nerve endings. without it, or if it's damaged, these things don't work very well and don't repair themselves very well.
4) the bubbles that are raised by the u/s on the skin from the sound waves are more dangerous than just blistering skin, as the fluid created from the heat is toxic and once it is released into the amnionic fluid is reabsorbed by the baby or into the placenta and therefore the mother to be disposed of by her body.
5) u/s is a useful tool, but shouldn't be used routinely. and any exposure should be short and to the point. repeated exposure shouldn't be routine, either.