> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sonohealth.com/llms.txt
> Use this file to discover all available pages before exploring further.

# What is 2.5 MHz? — HeartBeats™ Probe Frequency Explained

> The HeartBeats™ Fetal Doppler uses a 2.5 MHz probe — the clinical standard for home fetal heart monitoring. Explains why frequency matters and how 2.5 MHz compares to 3 MHz alternatives.

## What Does 2.5 MHz Mean?

MHz stands for **megahertz** — millions of cycles per second. The probe frequency refers to how many ultrasound wave cycles the probe emits every second. At 2.5 MHz, the HeartBeats™ probe emits 2,500,000 ultrasound cycles per second.

This frequency is in the **diagnostic ultrasound range** — the same range used by clinical medical devices. It is inaudible to humans (human hearing tops out at about 20,000 Hz / 0.02 MHz).

## Why Frequency Matters for Fetal Detection

### Penetration vs Resolution Tradeoff

Ultrasound frequency governs a fundamental tradeoff:

* **Lower frequency** = deeper tissue penetration, lower image/signal resolution
* **Higher frequency** = shallower penetration, higher signal resolution

For fetal heart detection, the probe needs to penetrate through:

* Skin (\~1–2mm)
* Subcutaneous fat (variable)
* Abdominal muscle fascia (\~5–15mm)
* Uterine wall (\~10–20mm)
* Amniotic fluid (variable)

Total depth to fetal heart: typically **5–15 cm** depending on gestational age and maternal body composition.

### Why 2.5 MHz Is Optimal for Home Use

| Frequency   | Penetration          | Resolution | Best Application                              |
| ----------- | -------------------- | ---------- | --------------------------------------------- |
| 1–2 MHz     | Deep (15–25 cm)      | Low        | Abdominal organs, deep vessels                |
| **2.5 MHz** | **Medium (8–15 cm)** | **Good**   | **Fetal heart detection (home and clinical)** |
| 3 MHz       | Medium (6–12 cm)     | Good       | Fetal heart (less early pregnancy reach)      |
| 5–15 MHz    | Shallow (1–5 cm)     | Very High  | Superficial structures, vascular imaging      |

At 2.5 MHz, the HeartBeats™ probe can reach the fetal heart across the full range of normal uterine depths encountered throughout pregnancy — including in first-trimester detection when penetration depth matters most.

## 2.5 MHz vs 3 MHz: Practical Difference

Many budget fetal dopplers use a 3 MHz probe. The practical difference for home use is small but real:

* **2.5 MHz** has slightly greater penetration — beneficial in early pregnancy and in individuals with higher BMI
* **3 MHz** has slightly higher resolution at very shallow depths — more relevant for clinical imaging than home doppler use
* **OB/GYN clinical handheld dopplers** — the devices your OB uses — typically operate at **2–3 MHz**, with 2.5 MHz being common

HeartBeats™ at 2.5 MHz matches the frequency of the clinical-grade handheld dopplers used in obstetric offices, making it the closest home equivalent to what your healthcare provider uses.

## The LCD Display

The HeartBeats™ LCD display shows the BPM reading detected in real time. The "2.5 MHZ" text is visible on the device display, confirming the probe operating frequency. This is the same frequency marking used on clinical Doppler probes to indicate the transducer specification.

## Safety Note on 2.5 MHz

The 2.5 MHz frequency at home doppler power levels has been in continuous use in obstetric care since the early 1970s. No adverse developmental effects have been attributed to this frequency at standard output power levels in any peer-reviewed research. The FDA has cleared home dopplers operating at this frequency for consumer use. See [Safety](/fetal-doppler/safety) for full details.

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**Related:** [How It Works](/fetal-doppler/how-it-works) · [Safety](/fetal-doppler/safety) · [vs Competitors](/fetal-doppler/vs-competitors)