When I was pregnant, I was surprised to encounter apps claiming they could detect a baby’s heartbeat using only a phone microphone. They were popular. They were marketed with confidence.
As a professional audio engineer, I knew immediately it wasn’t possible. A microphone measures changes in air pressure. A fetal heartbeat is not an airborne signal.
Still, I was 17 weeks pregnant, and in a mom group where everyone was recommending whatever had calmed their anxiety that week. I felt terrible (but wanted tacos), and had access to a professional studio.
So I did what any mildly sleep-deprived audio engineer would do. Not because I expected it to work, but because once you’ve pictured a U87 pointed at your stomach in the name of science, you can’t not follow through.
The Studio Test
The studio had an acoustically treated control room, a professional audio signal chain, and a mic collection that cost as much as a car. If there were an airborne signal to capture, this setup would have found it.
It didn’t.
What it captured clearly were stomach noises, fabric movement, HVAC, and the faint life of nearby production stages. When I tried recording my own adult heartbeat directly against my chest, I could eventually hear it – faintly – but buried beneath the rustle of an impromptu Foley session.
If a U87 through a GML into an SSL can’t isolate an adult heartbeat in a controlled studio, a mobile phone microphone pressed against a pregnant belly cannot detect a fetal one.
The issue isn’t quality. It’s physics.
Why a Microphone Can’t Do This
A microphone responds to changes in air pressure. When something creates sound in open air – a voice, an instrument, a footstep – it has caused pressure variations in the air. Those tiny fluctuations move a microphone’s diaphragm, which converts them into an electrical signal.
A fetal heartbeat generates mechanical motion inside the body. But that motion is occurring behind layers of tissue, fluid, muscle, and another body entirely. By the time any of that motion (vibration) reaches the surface, it is extremely weak in comparison to stronger surface and contact noises such as breathing, clothing movement, and simple body motion. Those sounds radiate into the air far more easily than the subtle internal movement of a fetal heart.
A Doppler monitor – the device doctors use to detect a fetal heartbeat – works on a completely different principle. It sends high-frequency ultrasonic waves into the body. Those waves reflect off moving structures like the heart, bouncing back to the montor. The device compares the outgoing ultrasonic wave to the returning one and converts the tiny motion-induced differences into an audible signal.
That’s why a phone microphone and a fetal Doppler aren’t different versions of the same tool. They’re built to detect entirely different things.
The Illusion of Detection
A baby’s heart rate in early pregnancy can be twice as fast as an adult’s. A baby is often in the 140–170 beats-per-minute range. And importantly, you cannot simply hold your ear to your own body parts and hear your heartbeat.
With that in mind, I downloaded and tested several of the heartbeat detection apps available at the time.
- Some produced no reading at all – which, given the physics, is what should happen.
- Other apps generated numbers at times.
- In one case, the app played back a steady heartbeat as the phone was placed at my abdomen and my chest.
That wasn’t detection – it was a simulated heart beat.
That’s scary when an app is presenting itself as a medical tool and not a game. If someone believes they are hearing a real fetal heartbeat, they may delay seeking medical care when something feels wrong. That’s dangerous.
What Actually Works
If someone wants to hear a baby’s heartbeat outside of a clinical setting, the technology required is different from a microphone.
A Doppler fetal monitor uses ultrasound and is typically able to detect a heartbeat around 10–12 weeks, depending on conditions. These are medical devices and are designed to be used by trained professionals. Some are available for home use, but it is important to speak with a healthcare provider before purchasing or relying on one.
Later in pregnancy – usually from 18–20 weeks onward – it may be possible to hear a fetal heartbeat with a stethoscope or a traditional Pinard horn, though this depends on positioning and experience.
If you are considering equipment, make sure it is an actual medical Doppler device that uses ultrasound and not a phone app that relies on a microphone. I’ve included examples of the devices I’m referring to below (for education purposes – not as a recommendation). Always consult your provider before using medical equipment at home.
- Stethoscope for Doctors/Nurses/Nursing School Students, Medical and Home Use
- Wooden Pinard Stethoscope For Midwives
- MINI-Doppler ES-100, Detect and Display Fetal Heart Rate
What This Experiment Actually Showed
We live in a time when software feels capable of almost anything. Phones and wearables can track heart rate, oxygen levels, sleep cycles, and movement. It’s easy to assume that if an app or device exists, the detection behind it must be real.
A microphone cannot become a Doppler monitor just because an app says it can.
When a product claims to monitor a pregnancy, that’s not a tech feature. It’s a medical claim. And medical claims deserve scrutiny.