Before you buy a home doppler
An app is not a doppler, and that is the whole point
A doppler sends ultrasound into your body and reads the echo. A phone only listens to what is already coming out. That single difference explains what a phone can do, what it cannot, and why the NHS tells you not to check on your baby with either one.
The engine, running on public recordings in your browser. Nothing is uploaded.
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 or a doppler first.
The difference is physical, not a matter of quality
People look for an app because a home doppler costs money and feels like a device, and they wonder whether the phone in their pocket does the same job. It does not, and better software will not change that, because the two are not doing the same thing.
A doppler is active. It transmits a beam of ultrasound into your abdomen and measures the shift in what bounces back off moving tissue. Movement is what it detects, which is why it can find a heart the moment it is beating hard enough to shift the beam. An app is passive. It records the sound that is already leaving your body and pulls the rhythm out of it, the same physics as a stethoscope with better arithmetic behind it.
| Home doppler | Firstsong on an iPhone | |
|---|---|---|
| Does it send anything into you | Yes. It transmits ultrasound and reads the echo. | No. It only records sound that is already leaving your abdomen. |
| Can it find a heart a phone cannot | Yes. That is what the transmitted beam buys you. | No. Some days there is nothing a microphone can lock onto. |
| Can it tell you your baby is well | No. A rate is one number at one moment. | No, and it says so on the screen instead of implying otherwise. |
| What happens when it fails | You keep hunting, and people often land on their own pulse without knowing it. | It reports nothing found, gives the usual reasons, and repeats the movements advice. |
| Can you check it before paying | No. | Yes. The same engine runs on this website on public recordings. |
What the NHS actually says about home dopplers
This is not our opinion and it is not marketing. It is three sentences on the NHS page about your baby's movements, quoted here in full.
Do not use a home doppler (heartbeat listening kit) to try to check the baby's heartbeat yourself. This is not a reliable way to check your baby's health. Even if you hear a heartbeat, this does not mean your baby is well.
The whole page is here, and it is worth ten minutes of your time whatever you decide about devices. Everything we build has to live inside that sentence, which is why Firstsong is sold as a way to hear your baby and never as a way to check on one.
The concern is reassurance, not radiation
Notice what the warning is about. It is about reliability, not about the ultrasound itself. The problem with a home doppler is not what it puts into you. It is what you do with what comes out.
A doppler finds something often enough to be believed and misses often enough to be useless as a check, and both outcomes send you the wrong way. Hearing something can turn reduced movements into a reason to wait until morning, which is the exact opposite of what every piece of guidance asks you to do. Hearing nothing can put you in a panic over a baby lying in an awkward position. An app is the same trap in a cheaper package, which is why this one is built to refuse and to say what a refusal means. What silence actually means is here.
Watch it turn one down
Three real recordings from open research databases, analysed in your browser. One is a baby, one is a mother, and one has nothing in it at all.
No account, no upload, no cookie.
So what is an app honestly for
Not for checking. For hearing. Those are different products that happen to share a screen, and the whole category gets into trouble by pretending they are one.
- The sound, in your own time. Twenty minutes in a quiet room with wired headphones, with nobody waiting on the appointment to end. That is what a scan cannot give you.
- A recording you keep. It stays on your phone and in your own private iCloud. There is no account and nothing is uploaded.
- A verdict you can argue with. It prints a rate only when independent windows agree, refuses anything that matches your own pulse, and tells you when the answer is nothing. The maternal trap is here.
What we measured, and what we have not
Before any of this had an interface, the engine was run against open databases on PhysioNet that anybody can download.
The negative set is the PhysioNet CinC Challenge 2016 heart sound set and the agreement figures come from the Shiraz University Fetal Heart Sounds Database. Both are open and both are downloadable, so none of this has to be taken on trust.
Those recordings were made with dedicated research sensors on the belly, not with an iPhone microphone. How much 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. A doppler does not have that problem, and saying so costs us nothing because we are not selling you a doppler.
If you have already bought one
Then you have not done anything terrible, and this page is not here to make you feel bad about a purchase. Use it for the sound if you enjoy it. Just do not let it into a decision. If movements change, the doppler does not get a vote and neither does an app. You call, and you call the same night.
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
Can an app replace a home doppler?
Not at the job of finding a heartbeat. A doppler transmits and a phone does not, so there will be days and positions where the doppler finds something and the phone gets nothing. What an app can do instead is tell you when it has found nothing, and name your own pulse when that is what it is hearing.
Is a home doppler dangerous?
That is not the question the NHS answers and it is not one we are qualified to answer. What the guidance says is that using one at home is not a reliable way to check your baby's health, and that hearing a heartbeat does not mean your baby is well. That is the part that matters for the decision in front of you.
Why can a midwife find the heartbeat when I cannot?
Training, a transmitting device, and knowing where the baby is lying before she starts. She also has the rest of the picture in front of her and is not doing it alone at midnight. None of that transfers to a phone on your own sofa.
Does Firstsong use ultrasound?
No. There is no transmitter in the product at any point. It is your iPhone microphone, a band pass filter and the decision logic, and it puts nothing at all into your body because it has nothing to put.
What should I do if I cannot feel the baby moving?
Call your midwife or maternity unit straight away, at any hour, and do not try to find a heartbeat first. The NHS page on movements says to call immediately and not to wait until the next day.
There is no version of this app, and no device sold for home use, that changes that instruction.
Keep reading
- What midwives warn about. the longer version of the reassurance problem
- What a baby heartbeat app is. the category taken apart, from the microphone to the decision
- Are these apps safe. what is and is not being emitted, in plain terms
- How to listen at home. the room, the headphones and where the phone actually goes
Last checked 2026-08-23