What is measured by an external ultrasound transducer to calculate fetal heart rate?

The correct choice relates to the measurement of reflected signals that exhibit a change in frequency, which is the principle behind Doppler ultrasound technology used in external fetal heart rate monitoring.

When the ultrasound transducer emits sound waves, these waves bounce off the moving structures within the fetus, particularly the heart. As the heartbeats and blood flows, the frequency of the reflected sound waves changes due to the Doppler effect. This change in frequency is directly proportional to the velocity of the fetal heart movement, allowing the ultrasound machine to calculate the fetal heart rate accurately.

Other methods mentioned, like reflected signals without frequency change, would not provide the necessary information about motion, thus failing to produce a fetal heart rate. Similarly, direct sound waves or magnetic resonance signals are not used in this context for fetal heart rate acquisition; fetal heart monitoring relies specifically on the shifts in frequency indicated by moving sound waves in response to fetal activity.