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How does the fetal heart rate monitor work?

The detector head sends out a high-frequency ultrasonic wave. The wave encounters various tissues on its way, which partially reflect it towards the head. The head receives ultrasounds, converts them into electrical signals, which, after amplification and filtration, are converted into a sound signal. The principle of operation of the detector is related to the Doppler effect, which consists in the fact that every moving object in the path of the ultrasonic wave changes its frequency. The change in frequency depends on the speed of the object. These objects are the heart of the fetus, as well as the flowing blood in the vessels of the mother and child.