The week by week answer
When you can hear a baby heartbeat at home
The short version is that a phone is a third trimester instrument. Everything we validated sits at week 28 and above. Underneath that line the public evidence runs out, so we make no promise there, and this page is about what that costs us and why we do it anyway.
A real third trimester recording, analysed in your browser, so you know what you are aiming for.
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.
Later than you want, because of physics
The date you are thinking of is the appointment where you first heard it, and that was a doppler. A doppler transmits ultrasound into your abdomen and reads the shift in what bounces back off moving tissue, which is why it works so early. A phone transmits nothing. It records sound that is already leaving your body, so it needs a heart big enough and close enough to be audible through everything in the way.
That gap is not a software problem and no update is going to close it.
What the public data shows from the third trimester on
Before any of this had a screen, the engine was run against open databases on PhysioNet. Here is what it found, split by how far along each recording was.
| 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 near term band is the strongest of the three, which is the opposite of what most people expect from a bigger baby and a thicker wall. The middle band is the weakest, on twenty one recordings, which is small enough that we would not bet a product on the dip being real.
Why nothing is published underneath that line
The same database holds four usable recordings below week 28. Four is not a sample. It is an anecdote with a file extension on it, and any detection rate calculated from four recordings would be a decoration rather than a measurement.
So Firstsong asks how far along you are and changes what it says to you. Earlier than the third trimester it tells you plainly that a phone is unlikely to find anything, before you start rather than after you have paid. Anything else would be invented.
The strangest chart in this category
We read 1,995 reviews across 18 apps in this category. 574 of them state how far along the reviewer was. Sorted by that, they look like this.
| Weeks stated in the review | Reviews | Five star | One star | Average |
|---|---|---|---|---|
| 4 to 12 | 150 | 63 per cent | 25 per cent | 3.79 |
| 13 to 19 | 163 | 41 per cent | 44 per cent | 2.98 |
| 20 to 27 | 115 | 13 per cent | 70 per cent | 1.90 |
| 28 to 42 | 146 | 12 per cent | 79 per cent | 1.62 |
Read the top row again. The best ratings in this entire category come from the stretch of pregnancy where a phone microphone has essentially nothing to work with, and the worst come from the third trimester, which is the only stretch where it has a real chance. Satisfaction runs exactly backwards to the physics.
There is really only one way that happens. An app that always produces a heartbeat sound will delight someone who has no way to check it and infuriate someone who does. By the third trimester there is a scan behind you, a doppler at every appointment, and a baby who kicks when you talk, so you know what the truth sounds like and you notice when an app is making it up.
The honest caveat is that this measures stated satisfaction and not detection. Nobody was standing over these reviewers with a reference recording. But it holds across three separate ways of extracting the week and across four years counted individually, and no reading of it flatters this category.
Check the engine yourself
Three real recordings from open research databases, analysed in your browser. One is a baby at 35 weeks, one is a mother, and one has nothing in it at all. Watch which one gets a number.
No account, no upload, no cookie.
What changes as the weeks go on
- The heart gets bigger and louder. More muscle moving more blood makes more noise, and noise is the only thing a microphone has.
- The baby settles. Once the head is down, the heart sits low and forward in a place you can predict, which is half the battle.
- Nothing changes about your own pulse. It stays the loudest thing in there at every stage, which is why the same mistake gets made in every week of pregnancy. The long version is here.
Three things that beat the week
Gestational age is the biggest single factor and it still loses to all three of these on any given evening.
- An anterior placenta. It sits between the microphone and the heart and muffles the whole thing. It is common, it is not a problem, and it makes this harder. What it does to the sound is here.
- Position on the night. Back to your spine, knees in the way, or simply turned over since this morning. It changes hour to hour.
- The room. Traffic, a fan, a washing machine two rooms away. Below a certain noise floor detection collapses to nothing at all. The whole setup is here.
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 a phone hear anything before week 20?
Not in any way we are willing to promise. The public evidence we validated against runs out underneath the third trimester, so we publish no number there and the app says so. If you try anyway and get static, that is the expected result and it means nothing about your baby.
I heard the heartbeat at my 12 week scan, so why not now?
Because that was a doppler, or a scan, and both of them send energy into your body and read what comes back. Your phone does neither. It is standing outside the room listening at the door, and it needs the sound to be loud enough to come through on its own.
Is week 28 a hard line?
It is a line we drew, and it is where the public data becomes thick enough to say anything with. Bodies do not know about it. Some people get a clear recording earlier and some get nothing at term, and neither outcome says anything about the baby.
Nothing at 35 weeks. Should I be worried?
Not about the silence itself. An app finding nothing is a statement about a microphone, a room and a position, and it is not information about your baby. What silence actually means is here.
Movements are the thing that matters. If you notice fewer than usual or any change in the pattern, call your midwife or maternity unit straight away, and do not go looking for a heartbeat first. The NHS guidance is here.
Why do the reviews look best at the earliest weeks?
Because that is when nobody can catch an app being wrong. It is the most useful chart in this category and it is the reason this product is built to refuse instead of to please.
Keep reading
- How to listen at home. the room, the headphones and where the phone actually goes
- Is it my heartbeat or the baby. the mistake almost everyone makes, and the arithmetic behind it
- Anterior placenta. why your belly sounds further away than everyone else's
- Do these apps work. what the reviews of this category actually say, counted
Last checked 2026-08-23