When I was pregnant, I was surprised to find apps that claimed they could detect a baby’s heart beat. As a professional audio engineer, I didn’t think it was even possible for a microphone to pick up sounds deep inside the body.

So, I decided to prove it by testing at a professional sound studio with over $100k of gear (tech details later) and also testing many apps on an iPhone. I was around 17 weeks pregnant at the time.

Long story short:

None of the microphones picked up a heart beat.

Using professional equipment, all I heard was a lot of stomach noises. I also tried recording my own heart beat which I eventually heard (faintly)… along with a lot of stomach noises.

It was not an easy process for a professional audio engineer to capture an adult heart rate with a microphone. It certainly was not as easy as pressing a microphone to the stomach or chest.

From there, I tried every baby heart rate monitor app I could find and with no surprise, none of them worked as they claimed.

Some apps gave no reading.
There are apps that falsely call themselves a “Fetal Doppler.” Unless you’re buying a doppler device that sends a signal to your phone, this claim is 100% false and deceptive (more on this later)
Some apps gave a false heart rate. One app even played back a heart beat that clearly wasn’t real. The way I knew it was fake is because it played that same heart rate whether I had the phone near my belly or my own heart. Plus, the heart rate was totally wrong for the age of my baby (a baby’s heart rate slows down as pregnancy progresses).

What’s scary about these false readings (especially the fake heart rate) is it could convince a mom nothing is wrong when there is in fact a problem. An app should never be used be used to replace the expertise of a medical professional. When in doubt, see a doctor.

What works to hear a baby’s heart beat instead of an app

First and foremost, always consult your doctor before buying or trying medical equipment.

It is possible to buy fetal doppler monitors online but technically these devices are prescription and designed to be used by trained professionals. (More on Dopper Monitors below.)

After 20 weeks, you could try a stethoscope, like the FriCARE Stethoscope.

Before electronic monitors existed, doctors used a device called a Pinard Horn to hear a baby’s heart rate, such as the AAProTools Pinard Fetal (Embryo) Stethoscope.

What is a Doppler fetal monitor?

What doctors use today is a Doppler fetal monitor. A baby’s heart is hidden behind a lot of physical layers. There’s both mom and baby’s skin and organs. Plus, there’s sounds coming within mom’s body (her own heart beat, blood moving, stomach growling, etc). It’s loud in there! Between all these layers (and a baby who might be moving around a lot) it’s not always easy to find a baby’s heart rate using a Doppler fetal monitor – even for a medical professional.

A Doppler fetal monitor uses ultrasonic sound waves, which are frequencies higher than humans can hear (think of a dog whistle – but higher). When the waves pass through skin and are reflected by moving objects (like a baby’s heart), the monitor picks that up and “converts” it to a sound (heard through a speaker).

Typically, an electronic fetal monitor can pick up a baby’s heart rate starting around 10-12 weeks. A stethoscope can’t register a baby’s heart rate til around 18-20 weeks.

While we can’t hear ultrasonic frequencies being emitted by a Dopper fetal monitor, babies in the womb may hear when the ultrasound is turned on and off.

Why are people drawn to apps to check a baby’s heart rate?

I totally get why these apps are appealing early in pregnancy. It’s worrying in the early weeks when you can’t feel the baby moving. It’s common to look to the web for a solution (which is probably how you landed here!) The problem is these apps just don’t work. It’s not possible.

In my own pregnancy experience, I found once I felt movement regularly (around 20 weeks) I didn’t have much desire for a doppler monitor anymore. I considered buying one many times before then! But, if I wanted to know my baby was ok, I’d just drink some orange juice and wait for the flutters or kicks. If there’s any doubt, a doctor visit is always going to be your best (and safest) way to go.

Technical Notes

The reasons a microphone can’t work

Microphones are designed to measure sound wave changes (vibrations) in the air. A Doppler monitor works without air (that’s why a doctor uses gel with an ultrasound – to help remove air bubbles). The mechanics of a microphone make it impossible to use as a fetal Doppler monitor. It doesn’t matter if it’s a professional microphone or a component in a telephone – it won’t work.

A real Doppler monitor works in a completely different frequency range than a microphone. The frequency range most software is designed to pick up is essentially the same as human hearing which is 20Hz-20kHz (20-20,000). A Doppler fetal monitor works at 3MHz (3,000,000).

Studio Equipment

The equipment at the studio I used to test included: GML 8304 mic pre, SSL AWS 948 board (for A/D), then digital signal into Pro Tools. I tested with 4 mic types: Neumann U87 (condenser), Sennheiser ME-66 shotgun mic, Sony ECM-77B (lav), and Audix D2 (dynamic mic). These mics range from $100-$3,000 USD.

The audio technology in a mobile device consists of a few parts: Microphone (typically MEMS), analog to digital converter (A/D), headphone amp, speaker. For all of these to fit in a handheld phone, all the components need to be lightweight, small and affordable. What this means is manufacturers are going to pick components based on size, cost, and practical use and not what’s best for sound quality. Because of that, the components in a consumer device (like a cell phone or tablet) will be lower quality than those in professional audio equipment.

The cost of components for Android and iPhones is projected to be under $30. That’s for everything. I can’t find data but would guess their microphones cost $5 or less. That alone should be enough to raise skepticism. If a $3,000 microphone designed for capturing sound can’t pick it up, why would a $5 component work?

Microphones can pick up frequencies much higher than 20kHz. The limitation with using those frequencies isn’t the microphone, though. It’s converting from analog to digital (see the Nyquist-Shannon sampling theorem). It also takes DSP to sample higher frequencies (in other words, it takes software memory to record). There’s no reason for consumer products to be taking in these frequencies we don’t hear.

I did look into whether mobile devices could send or receive ultrasonic frequencies. I was surprised to find they can and they do. Wired suggested they are being used for tracking purposes.

Do Baby Heart Rate Apps Work?

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