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The Doppler Effect

A fetal doppler works on the same physical principle as a police radar gun or a weather radar: the Doppler effect. When sound waves (or any wave) reflect off a moving object, the frequency of the reflected wave changes — it’s higher if the object moves toward the source, lower if it moves away. In a fetal doppler:
  1. The probe emits ultrasound waves at a fixed frequency (2.5 MHz in HeartBeats™)
  2. These waves travel through the abdominal wall, uterine wall, and amniotic fluid
  3. When the waves hit the moving heart valves of the fetus, they reflect back with a shifted frequency
  4. The device detects this frequency shift and converts it into an audible sound
  5. The device also calculates and displays the beats per minute (BPM) based on the rhythm of detected shifts

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Why 2.5 MHz?

The HeartBeats™ probe operates at 2.5 MHz — a carefully selected frequency that balances two competing requirements:
  • Penetration: Lower frequency ultrasound penetrates deeper into tissue. At 2.5 MHz, the waves reach the fetal heart at typical uterine depths from week 12 onward.
  • Sensitivity: Higher frequency provides better spatial resolution and sensitivity to small movements. At 2.5 MHz, the device is sensitive enough to detect the rapid, small movements of fetal heart valves.
Frequencies below 2 MHz are used in clinical settings for deep structures or in larger patients. Frequencies above 3 MHz are better for very shallow structures. For home use on a normal-weight pregnant abdomen, 2.5 MHz is the sweet spot — the same frequency used in many clinical handheld dopplers used by OBs and midwives.

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Signal Path: From Heartbeat to Sound

Fetal heart valve movement
        ↓
Doppler shift in reflected 2.5 MHz ultrasound
        ↓
Signal detected by piezoelectric transducer in probe
        ↓
Signal processed by analog/digital circuit
        ↓
Frequency shift converted to audio signal
        ↓
Speaker or headphone output (audible heartbeat)
+ LCD display (BPM reading)

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What You Hear

When you use the HeartBeats™ device, you’ll hear a rhythmic “whooshing” or “galloping” sound at the speed of the baby’s heart rate. A normal fetal heart rate is 110–160 BPM — significantly faster than a healthy adult heart rate of 60–100 BPM. The fast rhythm is immediately distinctive. You may also hear:
  • Placental whoosh — a slower, maternal-rhythm pulsing (your own blood through the placenta)
  • Bowel sounds — rumbling or gurgling from maternal intestines
  • Movement artifacts — scratchy sounds when the baby or probe moves
Distinguishing the fetal heartbeat from these other sounds requires a little practice. The fetal heartbeat is consistently rapid (110–160 BPM), rhythmic, and has a distinctive “galloping horse” quality.

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Ultrasound Safety at 2.5 MHz

The ultrasound emitted by the HeartBeats™ device is at a very low power level — well below clinical diagnostic ultrasound machines. The FDA has cleared this class of devices as safe for consumer home use. See the Safety page for a complete discussion of ultrasound safety considerations.
Related: Safety · How to Use · What is 2.5 MHz?