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2.5 MHz refers to the ultrasound probe frequency — 2.5 million sound wave cycles per second. It determines how deep the ultrasound waves penetrate tissue and how well they resolve structures at different depths. For home fetal dopplers, 2.5 MHz is the standard frequency used for reliable detection across most body types.

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At a Glance

Detail
MHz stands forMegahertz — millions of cycles per second
2.5 MHz means2,500,000 ultrasound cycles per second
Used inSonoHealth HeartBeats™ and most clinical handheld dopplers
AdvantageDeeper tissue penetration vs 3 MHz
Standard forHome and clinical fetal heart monitoring

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How Fetal Doppler Ultrasound Works

A fetal doppler probe emits ultrasound waves that travel through tissue. When those waves hit moving structures — like the fetal heart valves and blood flow — they reflect back at a slightly different frequency. This is the Doppler effect, and the device uses the frequency shift to detect movement and calculate heart rate in BPM. The probe frequency (2.5 MHz) determines:
  • How deep the waves travel before attenuating
  • The size of structures they can resolve
  • Their effectiveness through different tissue types

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Why 2.5 MHz Is the Preferred Standard

Home fetal dopplers need to detect the fetal heart through abdominal tissue, which varies significantly in depth based on the user’s body composition and gestational age. 2.5 MHz offers the best balance:
  • Deep enough to reach the fetus through varying amounts of tissue
  • Sensitive enough to detect small, fast fetal heart movements
  • Standard frequency used in clinical handheld dopplers worldwide
Higher frequencies like 3 MHz have slightly better surface resolution but less penetration depth — which can make them less reliable for users with higher BMI or in early pregnancy.

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2.5 MHz vs 3 MHz — Practical Difference

For most users in the second trimester, the difference is minimal. The key practical advantage of 2.5 MHz shows up in:
  • Users with higher BMI — 2.5 MHz penetrates deeper, making detection more reliable
  • Early detection (10–14 weeks) — the fetus is small and relatively deep; 2.5 MHz handles this better
  • Versatility — reliable across a wider range of body types

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Is This the Same Frequency Used in Clinics?

Yes. The 2.5 MHz frequency used in the HeartBeats™ is the same range used in handheld clinical Doppler monitors worldwide. The difference between a consumer and clinical device is not primarily frequency — it’s build quality, calibration standards, FDA oversight, and intended use.

Shop HeartBeats™ Fetal Doppler

2.5 MHz · FDA-Cleared · 2-Year Warranty · $79