The category, from the inside
What a baby heartbeat app actually is
A microphone, a filter and a decision. The first two are a weekend of work. The third one is the whole product, and it is where the category falls down. In a public corpus of 1,995 reviews of these apps, 41 describe getting a heartbeat from an arm, a sofa or a bed, and 31 of those 41 are one star.
Three real recordings, three verdicts, decided in your browser before you install anything.
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. If you notice fewer movements than usual, or any change in your baby's usual pattern, call your midwife or maternity unit now. Do not wait to try an app first.
A microphone, a filter and a decision
Every app in this category has to do the same three things. It opens the iPhone microphone, it throws away the frequencies where a heart cannot live, and then it decides whether what is left is a rhythm or just noise with a shape to it.
The first two steps are ordinary signal processing. Any competent developer can write them in a weekend, and most have. The third step is the entire product. Deciding well means being willing to answer no, and no is the one answer this category has trained itself never to give.
It listens, it does not transmit
A doppler is a transmitter. It sends ultrasound into your abdomen and reads what comes back off a moving surface, which is exactly why it finds a heartbeat that a phone cannot. An app sends nothing at all. The microphone picks up sound that is already leaving your body, the same sound a midwife hears through a Pinard horn.
That is the honest trade. Nothing is emitted, and there is also far less to work with. A phone listens through skin, fat, muscle, the uterine wall, fluid and whatever the room is doing behind you. The full comparison with a home doppler is here.
Why most of them disappoint
Read the reviews of the biggest apps in this category and the same two sentences come back over and over. It did not work. I think that was my own heartbeat. Both complaints have one root cause.
Your pulse is the loudest thing inside your abdomen, and at rest it sits at a rate that, doubled, lands in the middle of the range a baby lives in. An app that looks for the strongest repeating thing will find your heart, double it, and hand you a confident number in the fetal range. It looks like success. It is your own body.
A number over static
A blank screen reads like a broken app, so the screen fills the gap. Once a product decides that an empty result is a bug, every empty result turns into a guess with a decimal point on it.
Your own pulse, doubled
The rhythm is real and the arithmetic is real. The conclusion is wrong. This has an actual fix, which is to read your true pulse and refuse anything that matches it or its harmonics.
No way to check it
Most of these apps cannot be tested until after you have paid, so the only evidence you get is the number they choose to show you. An app that cannot be caught being wrong has no reason to be right.
The arm test, which parents worked out on their own
There is a check that has been circulating in pregnancy forums for years and it beats any review. Put the phone on your forearm, or on a cushion, and see whether it still finds a baby. Plenty of them do. Firstsong ships that test inside the onboarding, before the paywall, because it is the fastest honest thing we can put in your hands. The whole write up is here.
Watch an engine refuse
The third recording in the tool is real audio with no rhythm that holds. That is exactly the input that produces the arm and sofa reviews. This one declines, in your browser.
No account, no upload, no cookie.
What we measured before designing a single screen
The engine was written and validated against public research databases on PhysioNet, and it existed long before the first interface sketch.
The first two come from the Shiraz University Fetal Heart Sounds Database and the third from the PhysioNet CinC Challenge 2016 heart sound set. Both are open, both are downloadable, and you are welcome to run your own numbers on them.
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.
It is also why the app asks how far along you are. The week by week version is here.
What Firstsong does differently
- It can refuse. A rate appears only after independent windows agree with each other. Until then you get a signal clarity meter instead of a guess, which is the difference between a measurement and a decoration.
- It knows your heart. It reads your real pulse from your Apple Watch or your camera and rejects anything that matches it or its harmonics, then says so in words. The long version is here.
- It explains a no. When nothing holds it says that this on its own means nothing is wrong, gives you the usual reasons, and repeats the advice about movements.
- It can be checked first. The engine that ships in the app runs on this website, on public recordings, with no account and no install.
What it costs, since this category never says
Firstsong is not on the App Store yet, so nothing here is on sale. It is written down anyway so nobody has to dig for it later. Annual is 79.99 USD, around 1.54 a week. Weekly is 12.99 USD. No trial, no introductory rate and no discount if you try to leave. An app that convinces you it works costs the same to run as one that shows a number over static. A machine readable copy lives at pricing.md.
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
Do baby heartbeat apps work at all?
Some of it works and some of it is theatre. Amplifying belly sound into headphones is real and it is not hard. Turning that sound into a trustworthy number is the hard part, and it is the part the reviews say goes wrong. We took the category apart here.
Is this the same as a doppler?
No. A doppler transmits ultrasound and reads the echo. An app only listens. That is why a clinic doppler finds a heartbeat when a phone gets nothing, and it is also why an app puts nothing into your body.
Can an app tell me my baby is well?
No, and no app can. A heart rate is one number at one moment. It cannot rule out anything and it is no substitute for feeling your baby move.
The UK NHS guidance on movements is here, and it tells you plainly not to use a heartbeat listening kit to check on your baby yourself.
Why do I need headphones?
Because the phone microphone and the phone speaker are inches apart, so amplified belly sound played out loud goes straight back into the microphone. Wired is best. Bluetooth adds a delay that does not change the rate but does mean what you hear lags what is happening.
When does it launch?
There is no date. It ships when the listening is good enough on real bellies, and not before. If the phone measurement comes back badly, the email list gets told that instead.
Keep reading
- Hear the difference. the engine running live on three public recordings, one of which is nothing at all
- Is it my heartbeat or the baby. the most common question in this category, answered properly
- How to listen at home. the room, the headphones and where the phone actually goes
- When you can hear it. what the public data shows, and where it runs out
Last checked 2026-08-23