Are babies listening to us already in the womb?

How people learn language fascinates me for years and it all started by observing my oldest daughter slowly developing her linguistic skills while interacting with the outside world. Making funny but understandable errors, learning by trial and error…Now when there is a small baby growing in my belly, responding to touches and voices, I wonder how much it actually hears and how its future development is affected by it…

Why is a newborn calming down while laying on our chest? Can we improve kids’ linguistic or musical skills by prenatal exposure to foreign languages, by talking to them or playing music to them? Or is how much we talk to the kid affecting our bonding and connection to the baby? Maybe we cannot answer these questions or not all of them, but we can shed some light on what is happening in the womb, we can explore the life in the near darkness and see how the small brain is step by step developing and responding to the outer world…

When does the fetus start hearing?

“Knock, knock, do you hear me?” that is the question which I am sometimes asking my growing belly when sitting in the chair, wondering if it is already the time, when the baby will start hearing me and what it actually hears. What is going on inside my baby’s brain in the womb?

The same questions were already asked by many people before me. To find out, when the fetus can already hear us, Hepper and Shahidullah¹ made the following experiment: They exposed fetuses of different ages to various sounds and measured the responses of the baby by ultrasound. Via this method, they saw first responses already in the 19th week of pregnancy for sounds with a frequency of around 500 Hz. Later, the fetus starts to respond to lower frequencies, and even later babies start reacting to high frequencies. This is closely connected not only to the development of the fetus’ brain but especially to the development of the inner ear of the baby. Maybe you already heard about the cochlea, an organ in the inner ear, where sounds from the outside world are translated into electrical impulses. The brain in parallel learns to read and process these signals. Thus, the development of this organ is crucial to enable babies to hear all kinds of sounds.

Thanks to this experiment, we know, that the baby can ‘hear’ sounds of our body (such as the heartbeat or stomach rumbling²) as soon as at the 18th or 19th week of pregnancy. It is important that these sounds come directly from the mother’s body to the baby ear and thus they have high intensity and can be recognized by the immature inner ear of the baby. The sounds that the baby hears in the womb become familiar and pleasant to it. That is why the newborn can be soothed by listening to your regular heartbeat, sleeping calmly on your chest. It is the first sound it ever heard and it was accompanying it in the womb every day. Thanks to this, I will be able to enjoy the feeling of baby sleeping calmly on my chest…In this time, the baby brain starts to reorganize, and specialized areas for different types of sounds slowly develop⁴.

As the baby brain and ear matures, lower and lower intensity is needed to elicit a response of the baby (decrease of 20–30 dB — that is 7 to 10 times lower intensity). So if in the first months of the pregnancy you could be in any loud place without affecting the baby at all, in the last months of the pregnancy you can observe that the baby might become restless when you are in a loud place as it is already able to hear clearly all the sounds around. The inner ear seems to be fully matured at the end of the 8th month of pregnancy when the last relevant neuronal synapses still develop³.

In the 27th week of pregnancy, the baby can hear most of the sounds from the outside world [1]. Then it comes the time for the brain to start the quick development of specialized areas for learning the language, distinguishing individual sounds, words, voices, and languages. These areas can be already distinguished in the brain of the fetus which is around 30 weeks old⁵⁶.

Maybe you heard that each of our brain hemispheres is specialized in something else, for example, that one of our brain hemispheres is more emotional, while the other one is more logical. That is also the case for language — for most of the right-handed people language is processed mainly by their left brain hemisphere. What is interesting is that this specialization happens already before birth (e.g., some parts responsible for language processing are larger in the left hemisphere than in the right one, and we can observe higher sensitiveness of the left hemisphere to speech right after birth. Interestingly, the development of brain areas connected to language is actually slower in the left hemisphere than in the right one⁷,⁵. Why? We do not know. The fact that brain regions for language development are distinct already at birth might be a partial verification for Chomsky⁸ hypothesis, that there exists a universal innate grammar, which we just fill in with vocabulary and rules.

And so our developing baby gradually gets to the point when it can recognize individual voices and in the 32nd week of pregnancy, it begins to distinguish even vowels of your native language². As I have already mentioned, what is familiar to us feels pleasant, and therefore we can observe that newborns show a strong preference to the mother’s voice over other voices and prefers the native language to other languages⁹.

How loud is it in there? Sound and music in the womb.

“I know that you can finally hear now, but do you REALLY hear me?” Is another interesting question which I sometimes asked myself in the last weeks of pregnancy. You know, what sense it makes to read a fairy tale to a baby if my voice can’t reach it through all the body tissue and skin? Or should I avoid going to a concert because of the noise? Do we actually know how loud it is in the belly and which sounds come through to reach the fetus’s ear?

There are three ways to find out. The first way is very direct. You can just place a microphone or hydrophone inside the vagina close to the baby’s head and listen. You will hear a lot of background noise and body noise, all of which are highly attenuated¹⁰,¹¹,¹². Nevertheless, mother voice and external speech will still be clearly emerging above the background noise with well preserved prosodic characteristics (such as intonation, tone, stress, and rhythm). You will be able to recognize up to 30% of the words of the mother’s speech¹¹,¹². Although the baby inside the womb can hear all the people which talk close enough to the belly, it will get during its life in the womb most familiar with the mother’s voice, because there is a significantly better transmission of maternal voice than of other voices outside the womb (Querleu found out that there is 20dB attenuation of external voices and only 8 dB of mother voices¹¹. How can this be explained? It seems that the mother’s voice is transmitted also through body tissues and bones which enables kids to get more used to this voice.

This is another reason, why the mother’s voice has a calming effect on newborns and is more attractive to the newborn than other female voices. We can observe that various external auditory stimuli and voices, in general, make fetus to move more and increase their heart rate¹⁰, but the mother speech actually calms them down and leads to decreased fetal heart rate (this effect is the strongest when the speech is in low frequencies and around 70dB). On the contrary, newborns do not show any preference for the voice of the father compared to other male’s voices, even after 4–10 hours of exposure to the father’s voice after delivery. Spence¹³ went even further and found out that infants do not prefer maternal whispered voice over other voices.

The second way to find out what the baby in the womb hears is that we observe the fetus’s reactions (typically how much it moves) when exposed to various sounds. Using this method it was for example found out, in which week the fetus starts reacting to the music. The responses of the fetus were measured for tones and various vibration noises during quiet and active sleep and wakefulness of the fetus. This study showed that the baby starts to respond to tones from approx. 27th week of pregnancy. It also seems that it prefers to dance awake because bigger responses can be observed during wakefulness than during sleep. If you consider going to a metal concert in the last month of pregnancy, keep in mind that unborn babies might even start to cry inside the womb if exposed to a sudden loud noise.

The last way is that we observe the baby after birth and see if it reacts to the sounds which it heard in the womb differently then to any other. If yes, we might assume, that these sounds actually reached its ears already in the womb and thus it can remember them. So how is that?

How our behavior in pregnancy affects the newborn?

“Will you remember what I am telling you?” is the last question for my little one, who is still waiting to get out of the belly. This question might sound a bit ridiculous, but I would have really liked to know, how will all these sounds and talks around the belly affect the life and development of the language of my baby when it finally gets out of the womb.

As I already mentioned, researchers found out that familiar sounds such as adult heartbeat or mother’s voice¹³,¹⁰ might soothe a crying newborn [10]. Newborn babies exposed to the adult heartbeat (72 beats/s) were sleeping earlier and gained weight quicker¹⁶. Also, when they played intrauterine background noise to a newborn, it had a short term pacifying effect¹⁷.

It seems that also other sounds like music¹⁴, or soap opera theme¹⁵ can have the same effects. Feijoo¹⁴ measured kids at delivery and 4–5 days old and found out that kids were soothed by the music which their mother listened regularly during the last 3 months of the pregnancy way more than any other music. So it seems, that the answer to the question: “Will you remember what I am singing to you?” is definitely yes.

And I really like the phenomenon called fetal ‘soap’ addiction. It was described by Hepper in 1988¹⁵,¹⁹ who studied mothers who regularly watched a soap opera ‘Neighbors’ during the last weeks of pregnancy (some even twice a day). Maybe you can already guess what they observed for their babies after delivery. A crying baby who was ‘listening’ to the soap opera already in the womb, is more likely to stop crying when the program begins than the baby of a mother who was not watching it in her pregnancy.

To find out what newborns actually prefer, researchers often use an experiment that explores newborn behavior while sucking on a pacifier (non-nutritive sucking behavior). Basically they measure how much the baby is sucking, what are the pauses between sucking and frequency of sucking when exposed to different stimuli. So easy :). Using this method, it was found out that newborns prefer familiar stimuli. For example, DeCasper¹⁸ found out that 2–3 days old babies preferred a story that mother read 6 weeks prior to pregnancy to the one they never heard.

Recent studies also suggest that babies can distinguish the language to which they were exposed during pregnancy. Moon⁹, measured the sucking behavior of 40 one day old (approx. 30 hours) infants who were listening to vowels from the native and unfamiliar language in the nursery. They found out that babies were sucking longer during listening to the foreign language than for the native one, which indicates that they can differentiate between these two languages. One recent study even shows that when the baby is exposed to one language during pregnancy and adopted at birth by foreigners, it can afterward produce mother language sounds or distinguish some words from the language it heard only in the womb. To explore the reaction of these kids to different languages and avoid other effects like a change of the speaker, Moon used in another experiment²⁰ a bilingual woman who was talking to 2 day old kids of French and Russian mothers. These kids again preferred the mother’s language to the unfamiliar one and the same behavior was observed also for low-pass filtered (filtering out the high frequencies) versions, which kept only the melody (prosodic cues) of the languages. On top, kids who were from families speaking neither French nor Russian were not able to discriminate between these 2 languages²¹. These findings were also approved by measuring the neural activity of the brain. May et al.²² exposed 20 monolingual English 0–3 days old newborns to low-pass filtered sentences (keeping only intonation, tone, stress, and rhythm of the voice) of forward played English (familiar language), forward played Tagalog (unfamiliar language), and backward played English and Tagalog (non-language) and measured their neural activation by NIRS (near-infrared spectroscopy). They observed different activation patterns of both hemispheres for the familiar and unfamiliar language. This indicates that exposure to the language in the womb influences the brain responses of the baby to the native language after birth.

But it is possible that they actually learn even more from their prenatal experience as they might become familiar with the melody of the languages heard in the womb (‘prenatal prosodic bootstrapping’), which might be crucial for later language acquisition²³.

The auditory and language system as a whole continues to develop also after a birth — e.g., hearing develops for another 5–6 months after the birth and language areas keep developing for years, so it needs continuous stimulation through speech, music, and other sounds, otherwise we might miss the important times (so-called sensitive periods) for its development which might lead to its underdevelopment, as observed for linguistically or socially deprived ‘wild kids’…but that is a topic for some other time…

So if you can, talk to your kid, play nice music around and let it learn all those sounds which will later feel pleasant to your newborn and calm it down. The home which you are creating for the baby is full of sounds, which are reaching it already during the whole last trimester of the pregnancy.

References

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