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Doppler radar is used to see a fetus. In some woman the fetal heart can be detected as early as 8-12 weeks.Fetal dopplers that employ a 2 mghz probe tend to pick up the fetal heart rate earlier than 3 mghz fetal dopplers as they have a more defined beam which penetrates deeper. These probes send a series of short sound pulses into the body and pauses between each pulse to listen for the returning sounds. The Fetal Doppler then determines the direction and depth of each returning sound and coverts this into an audible output. [| Click here.]

Doppler radar works by sending a beam of electromagnetic radiation waves, tuned to a precise frequency, at a moving object.When the electromagnetic radiation wave hits the moving object, it "bounces" back toward the source, which also contains a receiver as well as the original transmitter. The radar frequency is different when it comes back, and from that difference the radar gun can calculate object speed.The shorter the wave on the bounce back the faster a car is going. This is used in radar guns. Seen below.