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Joined Mar 2017
It just does!
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Doppler fetal heart monitors are actually prescription medical devices, even though some websites are unethically selling them over the counter. They're in the same category as the ultrasound machine found at your doctor's office. According to the FDA, both Doppler fetal heart monitors and ultrasound machines are:

"Prescription devices designed to be used by trained health care professionals. They are not intended for over-the-counter sale or use."

"Ultrasound can heat tissues slightly, and in some cases, it can also produce very small bubbles (cavitation) in some tissues."

The long-term effects of tissue heating and cavitation are not known. Therefore, ultrasound scans should be done only when there is a medical need, based on a prescription, and performed by appropriately-trained operators.