In terms of physics, Sound waves (including infra and ultra) are not as intense as many other wavelengths. Light is more intese than sound!! Sound waves are not dense enough to cause harm any more than normal light (UV is the highest level of light and that is what the damage starts at) If you shone an extremely bright light on your belly I think you would have as much chance of injuring the baby as you would putting a doppler on. There is very little risk identified because there is very little risk!! An Xray which is the way twins used to be determined is more harmful than an US so I am happy to have US and doppler as often as I need / want to.
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