iPhone, third trimester, not on the App Store yet
You should be skeptical of any app that says it can hear your baby
So we built ours to be checkable. The engine that decides what you are hearing runs on this page, on three real recordings, before you install anything and before you pay anything.
Three public recordings, three verdicts, decided in your browser. Nothing is uploaded.
Firstsong is not a medical device, not a doppler and it does not use ultrasound. It cannot tell you that your baby is well. If you notice fewer movements than usual, call your midwife or maternity unit now. Do not wait to try an app first.
The problem is not that these apps are hard to build
It is that the honest answer is often no, and no does not sell. So the category learned to show a number anyway. You put the phone on your belly, you hear static, and the screen still says 113 bpm. The reviews of the biggest apps in this category are full of the same two sentences: it did not work, and I think that was my own heartbeat.
Firstsong is built the other way round. The engine was written and measured against public research data before a single screen was designed, and it was built so that it can refuse. When nothing holds, it prints nothing and tells you why.
What it actually does
Listens, does not transmit
It is a microphone, not a doppler. Your iPhone picks up the sound already coming out of your abdomen, filters it to the 20 to 200 Hz band where heart sounds live, and amplifies it into your headphones. Nothing is sent into your body because nothing is emitted.
Prints a rate only when it holds
A number appears after at least 30 seconds of agreement across independent windows. Until then you see a signal clarity meter, not a guess. This is the difference between a measurement and a decoration.
Names your own heart out loud
Your pulse is the loudest thing in your belly, and at rest it can echo into the range a baby lives in. Firstsong reads your real pulse from your Apple Watch or your camera and refuses the match, then says so in words: that steady rhythm is yours.
Says nothing when there is nothing
Some days a phone will not find it. Anterior placenta, position, a noisy room, the wrong week. Firstsong says today it is not there and that this on its own means nothing is wrong, then repeats the movements advice.
See it refuse
The third recording in the tool is real audio with no rhythm that holds. Apps in this category do print numbers over audio like that: in a public corpus of 1,995 reviews, 41 people describe getting a heartbeat out of an arm, a sofa, a bed or a floor. Watch this one decline instead.
Runs in your browser. No account, no upload, no cookie.
What we measured, and what we have not
Before any interface existed, the engine was run against public databases on PhysioNet, the ones any researcher can download and any reader can check. These are our numbers on that data.
Here is the part nobody in this category writes down. Those recordings were made with dedicated research sensors on the belly, not with an iPhone microphone. How much of that survives a phone pressed against real skin is the one number we do not have yet, and we are not going to pretend otherwise until we do.
That is also why the app asks what week you are in and adjusts what it promises. Below week 28 the public evidence we validated against is four recordings. Four is not enough to promise anything, so we promise nothing there.
| Gestational age | Recordings | Rhythm found | Error against CTG |
|---|---|---|---|
| 28 to 34 weeks | 20 | 90.0 per cent | 3.52 bpm |
| 34 to 37 weeks | 21 | 85.7 per cent | 2.57 bpm |
| 37 to 42 weeks | 75 | 93.3 per cent | 2.47 bpm |
The arm test, which the internet already invented
Parents in this category work out on their own that you can catch a lying app in about thirty seconds. Put the phone on your forearm, or on a cushion, and see whether it still finds a baby. Plenty of apps do.
Firstsong ships that test inside the onboarding on purpose, before the paywall, because it is the fastest way to show what kind of app this is. On an arm you get a rate in the adult range and the app says so. On a cushion you get nothing and the app says nothing. We wrote the whole thing up here.
What it costs
Firstsong is not on the App Store yet, so nothing below is on sale. It is written down here so nobody has to dig for it later.
| Plan | Price | Free trial |
|---|---|---|
| Annual | 79.99 USD per year, about 1.54 per week | None |
| Weekly | 12.99 USD per week, about 675 per year | None |
No trial, no introductory rate, no discount if you try to leave. Instead the first listen inside the app is real and complete, with the noise check and the maternal reference running, and you decide after it. Restore purchases sits on the paywall and in settings, and a machine readable copy of this table lives at pricing.md.
It is more expensive than the apps you have already tried, and we know what that reads like. The honest version is this: an app in this category that convinces you it works, and an app that shows a number over static, cost the same to run. Only one of them is worth anything, and it is not the cheap one.
One email, the day it opens
Firstsong is not on the App Store yet. Leave an address and you get exactly one message when it is, with the price on the first line. No newsletter, no drip, no partner offers. You can reply to that email and a person reads it.
Stored on our own server, never sold, never shared. Your IP is hashed, not kept. Nothing about your pregnancy is asked for or stored.
Where to go next
- Hear the difference. the engine, running live on three public recordings
- Is it my heartbeat or the baby. the single most common question in this category, answered properly
- When can you hear a baby heartbeat at home. what changes week by week and why 28 is the line
- Anterior placenta. why your belly sounds further away than everyone else's
- All the answers. every question page in one place
- Longer reading. the pieces that take more than a paragraph
Common questions
Is this a doppler?
No. A doppler sends ultrasound into your body and listens to what bounces back. Firstsong sends nothing. It is a microphone and a filter, the same physics as a stethoscope with better maths behind it. That is also why it can fail where a doppler would not.
Can it tell me my baby is fine?
No, and no app can. A heart rate is one number at one moment. It cannot rule out anything and it is not a substitute for feeling your baby move.
If you notice fewer movements than usual, call your midwife or maternity unit straight away. The UK NHS guidance on movements is here and it does not mention apps for a reason.
Why does it need headphones?
Because the phone microphone and the phone speaker are inches apart, and playing amplified belly sound out loud feeds straight back into the microphone. Wired headphones are best. Bluetooth adds 150 to 300 milliseconds, which does not affect the rate but does mean what you hear lags what is happening.
Why is there no free trial?
Because a trial that auto renews into a yearly charge is how this category earns its worst reviews. Instead the first real listen happens before you pay, inside the onboarding, and it is the same listening the paid app does.
Does anything leave my phone?
No audio, no heart rate, no gestational age. Recordings live on your device and in your own private iCloud. There is no account and no login. This website has no cookies and makes no third party requests, which you can check in your browser network tab right now.
When does it launch?
There is no date. It ships when the listening is good enough on real bellies, not before. If you want to know the day it happens, the email box above is the only place we will announce it.
Last checked 2026-08-23