iPhone specifics
What makes this an iPhone app, and not yet an Android one
The interesting part of listening to a belly with a phone is not the app. It is which microphone you get, and what iOS has already done to the sound before any app sees it. On iPhone both of those are knowable, and that is the whole reason we started here.
The same engine the app ships with, running on three public recordings in your browser.
Firstsong is not a medical device, it is not a doppler and it does not use ultrasound. It cannot tell you that your baby is well and it cannot rule anything out. If you notice fewer movements than usual, call your midwife or maternity unit now. Do not wait to try an app first.
Why iPhone first
One microphone chain we can characterize
A small number of models, a small number of microphone layouts, and the same audio stack across all of them. On Android the same code meets hundreds of microphone and processing combinations, each one doing something slightly different to the low end.
A documented way to turn the processing off
iOS lets an app ask for the input with call processing disabled. That single request is the difference between hearing a heart and hearing a clean, empty recording.
A real pulse on the wrist
Apple Watch gives the app your actual heart rate from a separate sensor, which is what lets it refuse a rhythm for being yours instead of guessing from the same audio.
Private storage without a server of ours
iCloud already gives you encrypted personal storage under your own Apple Account. We can save your recordings there and never hold a copy, which means there is no account to create and nothing of yours on a machine we run.
None of that makes Android worse. It makes the same app a different measurement problem there, one we would have to work through device by device. Shipping it badly would be worse than not shipping it, and this category already has enough apps that say yes to everything.
Where the microphone is, and why the phone goes that way up
Every recent iPhone has several microphones. There is a group on the bottom edge beside the charging port, one at the top of the front near the earpiece slot, and one on the back beside the cameras. For plain recording, iOS normally hands an app the bottom edge one, which is why the phone goes port down and flat against skin.
| Your iPhone | The microphone you want against your belly | What tends to block it |
|---|---|---|
| iPhone 15, 16 and newer, with USB-C | Bottom edge, in the row of holes to one side of the port | A deep case lip, or a finger curled under the phone |
| iPhone 12 to 14, with Lightning | Bottom edge, same place, to one side of the port | The same, plus a card wallet or a grip sitting low on the back |
| iPhone SE | Bottom edge beside the port, on a smaller body | Your own palm, most of the time |
This matters more than it sounds. Cover that microphone with a case lip and the app is listening to the room instead of your abdomen. Two minutes without the case is worth more than any setting.
The setting that eats a heartbeat
An iPhone can hand an app its microphone in two very different states. One is tuned for a phone call: noise suppression, echo cancellation and a high pass filter, all designed around speech, which starts well above 300 Hz. The other hands the input over with that processing switched off.
A heart lives between 20 and 200 Hz. The call optimized path treats everything down there as rumble to be removed, because on a voice call that is exactly what it is. Point it at a belly and it quietly deletes the only thing you came for, then gives you a clean recording of nothing. Firstsong asks for the other path every time, which is also why the app cannot run inside a call, a video call or a screen share.
You can hear this for yourself in three minutes. Record thirty seconds of your abdomen in Voice Memos with any enhancement switched off, then record the same thirty seconds inside a voice call app, and listen to both on wired headphones. The second one is thinner at the bottom, and that missing bottom is the band a small heart lives in.
What the engine was measured on
Those recordings were made with research sensors on the belly, not with an iPhone microphone against real skin. How much of that survives the move to a phone is the number we do not have yet, and it is why there is no launch date on this site. When we have it, it goes on this page whatever it says.
Watch it refuse before you trust it
Three public recordings, analysed live in your browser by the same engine the iPhone app runs. One is a baby, one is a mother, and one has nothing in it at all.
Nothing is uploaded. No account, no cookie, no request to another domain.
Apple Watch, used as a witness
Your own pulse is the loudest thing inside your abdomen, and at rest it sits at almost exactly half of a typical fetal rate. That is the trap that produces the most common complaint in this whole category, and doubling is such a tidy piece of arithmetic that the wrong answer looks perfect.
So the app does not guess your pulse from the same audio it is trying to judge. It reads it from your Apple Watch, from a separate sensor, and refuses any rhythm that matches it or an exact multiple of it. Then it says so in words rather than showing a blank screen. Without a Watch it reads your pulse from the camera and the flash instead, which is slower and works fine. The whole trap is written up here.
Where your recordings live
On your iPhone, and in your own iCloud if you leave that on. There is no account, no login and no server of ours in the path, so there is no copy for us to lose, sell or hand over. This website behaves the same way: no cookies and no third party requests, which you can check in your browser network tab right now. What the recorder saves and the privacy page have the detail.
What an iPhone cannot fix
- An anterior placenta, which puts a cushion of tissue between the heart and the microphone. What that changes.
- Where the baby is lying, which moves day to day and can put the chest facing away from the surface entirely.
- The noise floor of your room, which no processing recovers once the recording is made.
- How far along you are. Below week 28 the public evidence behind this engine is four recordings, which is not enough to promise anything.
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.
Common questions about iPhone
Which iPhone do I need?
Any recent one. The limit in this category has never been the processor, it is your abdomen and the quiet of your room. What matters far more than the model is taking a thick case off and using wired headphones.
Do I need an Apple Watch?
No. The Watch is the fastest way to give the app a trustworthy reading of your own pulse, but the camera does the same job with the flash on. What the app will not do is skip the check.
Will there be an Android version?
Not soon, and there is no date. Doing it properly means characterizing a great many microphone and processing combinations, and doing it improperly means shipping an app that says yes to everything. That is the thing we are trying not to build.
Does it work with AirPods?
Yes, with 150 to 300 milliseconds of delay between the sound happening and you hearing it. The heart rate is unaffected because it is computed on the recording. A wired earbud removes the lag, which helps while you are hunting for a spot.
Why can I not just use Voice Memos?
You can, and it is a good experiment. What Voice Memos will not do is filter to the 20 to 200 Hz band, build the envelope that makes the quiet second sound survive, reject a rhythm for being an exact multiple of yours, or refuse to print a number when nothing holds.
Does anything leave the phone?
No audio, no heart rate and no gestational age. Recordings stay on the device and in your own iCloud. There is no account, so there is nothing on our side to leak.
Keep reading
- What it sounds like. the double knock, and the four things people mistake for it
- Recording and sharing. what a saved listen contains and where it lives
- Is it my heartbeat or the baby. why the Watch is in the loop at all
- Doing it at home. position, pressure and the quiet room
Last checked 2026-08-23