Smiling, laughing, loving — what is behind the smile of our baby?

Smiling baby trying to make his mother to laugh back to him.
Social smiles appear earliest after 4 weeks of life. Kids try to make parents smile back as much as possible.

Smiles, laughter, giggles — as soon as the baby’s face lights up into a guileless smile, even the grumpiest passer-by melts and has to smile. Smiles are the reward for parents for all those sleepless nights and the manifestation of the connection which we yearn for from the first day of childbirth.

A smile tells us that everything is fine.

A smile makes us laugh back.

The smile makes us feel good.

When can a baby really start laughing? And why are they smiling?

What happens in the brain of a child and an adult when they laugh?

Does it have any effect on the development of the baby in the uterus and on the newborn, whether his mother often laughs or is depressed?

How can robotics help us to understand the effects smiles have on us?

Rest of the story can be read on Medium:

First kicks and yawns – how our baby starts to move around the womb?

Right after the delivery we take our babies and pack them into a cloth that holds their hands and legs close to their bodies, to let them feel hugged and safe. And so we tend to forget that they can already move, that back there in the womb, they were moving, touching, listening, observing. We greet every move of their hands and legs with huge applause. We observe how they start pseudorandomly move their hands around, kicking legs when they want to reach a toy as they are exploring options of their bodies. Like every mum, also I can spend many hours just watching how is the little baby exploring the world around. Slowly, as they touch by their hands and legs surrounding objects or themselves, they start to move more and more intentionally, and we focus on every step of their development with incredible attention. All this seems to be a continuous development with the starting point at delivery. We tend to forget about their first kicks in our belly, by which they were letting us know that they are already there, about those long nine months which they spent exploring the womb. How it was in there? How often they move, how important is their motor development inside the belly for their future motoric development at all?

Compared to other infrahuman primates (this means chimpanzees, apes, and others) human babies have delayed development [1, 2]. This means that for many weeks after the birth, the baby actually does not change much its behavior compared to what it was doing inside the womb. It is still not able to care for itself and needs long term support from the mother. It seems that there is a central movement pattern generator [2] in the brain, which creates various movements (either general movements of the whole body or isolated movements of the individual limbs) and this same pattern generator is serving the body both before and after birth [3]. This gives us an amazing option to see, how was our precious bundle behaving inside our belly – which movements it used to do, how it sometimes accidentally touched its head by its hand, how it was sucking the amniotic water, how it was breathing, yawning, swallowing, kicking,… It takes up to 3 months after birth when these mainly random predefined motor patterns change to more intentional goal-directed movements, that are guided by the sensory system (which was also not fully developed in the first weeks after the delivery so the baby had to wait a bit longer to be able to make use of it) – and the baby can finally reach for a toy by hand or rotate in its direction…

Development of the motoric system

The motoric system is one of the first systems in the fetus’s body which starts to develop. It was shown that it is crucial for brain development that the motoric action provides an appropriate change in the sensorial world (e.g. we can feel that we touched something or we can see that something moved, etc.) (see e.g. Held and Hein [4] who performed a classic experiment with kittens). Thanks to modern ultrasound devices we can observe gradual changes in human fetus development in detail. First longitudinal studies were performed in the 1980s by Prechtl [5] and uncovered till that point many unknown phenomena about that secret world “inside”. From the studies of the motor patterns, we can nowadays observe also the brain connectivity of the fetus and see for example that good connectivity of some regions (prefrontal cortex, motoric network, supply motor regions, and temporal lobes) in the fetus brain is very important for later post-term motor development of the kids (Thomason, 2018). In this study, they found out that the connectivity also differs between female and male fetuses and discusses that this might be related to the later sex differences between infants. There are studies that show that female infants are better in fine motor skill development, while males in gross motor [6] or those female infants show more coordinated arm movements [7].

When do individual movements start?

As soon as in 8 weeks of pregnancy the fetus starts to move sideward its head [8]. These first body movements are followed by generalized movements of the whole body in 9-10 weeks – the whole body is either moving slowly in a sequence (general movements) or all limbs, trunk, and neck are moved by quick phasic movements (startles) [9]. It takes another week or two when we can (between 10-11 weeks) observe isolated local movements of one arm or leg. Isolated movements of some parts of the body are way more difficult to produce than whole-body movements both for the fetus as for the newborn. Teaching individual brain regions of the motor cortex to specialize for moving individual body parts will be under development for many years so it is no wonder that it is easier to generate in the fetus brain activity responsible for the whole-body movement. Just imagine when someone asks you to move one of your toes on your feet. Can you manage?

MRI image of baby kicking
Above, an animation made from MRI scans showing fetal kicks at various stages of development.
(Image: © Stefaan W. Verbruggen, et al./Journal of the Royal Society)

In the 10 weeks of pregnancy come hiccups. These movements are caused by repetitive short contractions of the diaphragm, which might last for several minutes and as you well know, they can be pretty annoying. Sometimes they might be so strong that they move the whole fetus in the womb.

If you wonder when the baby can already move its head to sign “no” or “yes” to your question by moving head back and forth or rotating it from side to side, it is actually as soon as in 11 weeks of pregnancy. At that time, the head becomes mobile and can move forward, backward, and rotate. It means that now we have to just find the way how to let the baby know that it can use it for communication with us and by observing the womb via ultrasonic devices, we can see if it likes or dislikes what we do 😊. What is great is, that this general pattern generator for limbs and head movements might create at this point accidental hand-eye contacts. These are totally not intentional, but still, provide important information for the fetus. These are the first self-contacts that enable later self-awareness of the baby and a better understanding of its own body. Same as we tried in our research where the robot was trying to touch itself to find out the length of its arms and torso as well as the position of its eyes (cameras) (Stepanova et al. [10], ).

In 11-12 weeks we can observe periodical breathing of the fetus and in 12 weeks we see stretches and yawns. It might impress you to observe your newborn yawning as it seems so mature. The fact is, that these movements which appear as soon as in 12 weeks of pregnancy will keep the same form and pattern for the whole life. Cool, right?

Sucking. You might be impressed by the newborn sucking reflex and then the strength of the sucking itself (at least some of the kids, some others need a lot of stimulation and their sucking is very weak). How quickly it finds the place to suck and how much it hurts when the baby actually sucks accidentally in the wrong place. No wonder, it has good training also for a few months before it comes to the world. After 12 weeks of pregnancy, the baby starts to drink the amniotic fluid, that is connected to first sucking and swallowing movements. As the sucking might happen in the womb all the time, after delivery, the baby will have to learn to connect this sucking behavior to feeding occasion – yet some of the kids would love to continue in the “sucking all the time” behavior 😊. Sucking reflex itself (meaning that the baby starts sucking as it gets nipple or something else to the mouth) develops around 32 weeks of pregnancy. (

Animation of the development of the fetus in the womb. You can nicely see different types of movements starting from sideward bending of the head, generalized as well as isolated movements, sucking, etc.

At around 20 weeks, the baby starts to prepare for the outside world and first slow eye movements (20 weeks) followed by rapid eye movements (22 weeks) develop. [11]

As the fetus is growing and space is getting restricted, most arm movements end up not far from the face, and the amount of hand-face contact increases [12]. The first movements are mainly unintentional, later there are also more and more non-reflexive responses to sensorial inputs – like to sounds, light (response to disturbances [13] or movements towards the source [14] (see also my earlier blog post about the language development), response to the maternal touch of the abdomen by an increase of arm, head and mouth movements [15] as well as changes in fetal heart rate. But still, most of the movements are only ‘‘motor babbling’’ – the spontaneous activity of the fetus.

By about week 21, the fetus begins to develop a regular schedule of movement [16] The startle reflex is present in half of all fetuses by week 24 and in all fetuses by week 28.

After the birth comes with sensory inputs also new options and new movements. The baby must deal with the gravity, it has to learn how to deal with the lung ventilation and so we can observe sneezing and coughing and so on.

The motor cortex of the fetus

It is believed [12] that the fetus is creating connections between motoneurons and muscles via these spontaneous movements when via general and isolated movements it receives various types of stimulations. In this way, specialized areas in the brain that are created to operate given body parts might be created. When individual body parts touches another body part or the outside world as well as when an outside stimulus triggers some sensation, all of these contributes in the creation of new neural connections and enable the fetus later to repeat any movement. You can see research about creating these somatosensorial maps in the brain of the humanoid robot ( , Hoffmann [17]).  

How often do babies change their position in the womb? And should you be worried if they do not kick enough?

I guess that every mother goes at some point through that scary moment. Sitting on the sofa and thinking that the baby did not move for too long. Starting to worry, counting seconds, getting scared if everything is ok with the baby. Should I call a doctor? Should I worry? What is wrong with this baby? Or when you come to the monitor before delivery and you everybody around gets “a nice” monitor immediately, only you are the one staying there longer, breathing deeply, sisters changing your position that the baby finally moves…It is easy to get stressed.

Up to 16 weeks of pregnancy the frequency of movements gradually increases and around 20 weeks, calm periods (without any type of movements like general or isolated movement, hiccups, mouth movements, etc.) are very short (maximum 13 minutes, de Vries et al., 1985). One of the cool things about all those types of movements mentioned above is that our baby can change its position in utero. What is interesting, that this change in position happens way more frequently in the first half of the pregnancy (up to 25 changes per hour) than in the second half. So, when we can finally feel the first baby kicks (at about 20 weeks of the pregnancy), changes in position are way rarer [2]. It is not that surprising if you consider that the baby does not have in the womb that much space anymore. It is believed, that this decrease in frequency is also connected to the emergence of inhibitory cortical influences [12]. Second or third children may have more stretching room in the womb than first babies because a woman’s uterus is bigger and the umbilical cord longer after her first pregnancy. These children usually get more motor experience in utero and tend to be more active infants [18]. Keep in mind that there are huge differences in movements and their strength. There are studies, which found out that boys might be kicking more than girls [19] with higher average movements at 20, 34, and 37 weeks. Anyway, the study was done only on 37 babies, which is really not a relevant sample ( ). When we go for a monitor, you can see many mothers discussing if they should eat or not before it to make the baby move. What was found out is, that most of the movements of the fetus are not connected to the amount of glucose in the mother’s blood (it does not matter if she is after or before meal)? The only exception is breathing movements of the fetus which are easier observable after the meal [2].

So you do not have to be worried if your baby kicking patterns are changing if you feel its kicks some weeks later or earlier if sometimes it is kicking more and sometimes less, especially if you are distracted by your own activity. Anyway, by week 29, you should be feeling your baby moves well. On average, you should feel some type of their movements (kicking, rolling, stretching, even light movements count) at least 10 times within 2 hours [20]. If you do not get these 10 kicks within 2 hours while being in a calm environment resting, you should change position, wait an hour or two, calm down and try to count once again. If you do not count a reasonable amount of movements even then, then it is a proper time to call your pediatrician, to make sure that everything is ok.

There is much more to tell about the motor development of the baby in the womb, especially about its brain development, but let’s continue some other time. There are still many secrets even when we have nowadays very good tools to look inside the womb. What we know for sure is that the movements of the baby in the womb (and all the sensorial inputs it gets thanks to them) are very important for its future development.

[1] Hadders-Algra, Mijna. “Early human motor development: From variation to the ability to vary and adapt.” Neuroscience & Biobehavioral Reviews 90 (2018): 411-427.

[2] Einspieler, Christa, Peter B. Marschik, and Heinz FR Prechtl. “Human motor behavior: Prenatal origin and early postnatal development.” Zeitschrift für Psychologie/Journal of Psychology 216.3 (2008): 147.

[3] PRECHTL, HF R. “Continuity and change in early neural development.” Clinics in developmental medicine 94 (1984): 1-15.

[4] Held, Richard, and Alan Hein. “Movement-produced stimulation in the development of visually guided behavior.” Journal of comparative and physiological psychology 56.5 (1963): 872.

[5] Prechtl, Heinz F., ed. Continuity of neural functions from prenatal to postnatal life. No. 94. Cambridge University Press, 1991.

[6] Touwen, Bert CL. Neurological development in infancy. London: Heinemann, 1976.

[7] Piek, Jan P., et al. “Limb and gender differences in the development of coordination in early infancy.” Human movement science 21.5-6 (2002): 621-639.

[8] Lüchinger, Annemarie B., et al. “Fetal onset of general movements.” Pediatric research 63.2 (2008): 191-195.

[9] De Vries, Johanna IP, G. H. A. Visser, and Heinz FR Prechtl. “The emergence of fetal behaviour. II. Quantitative aspects.” Early human development 12.2 (1985): 99-120.

[10] Stepanova, Karla, Tomas Pajdla, and Matej Hoffmann. “Robot Self-Calibration Using Multiple Kinematic Chains—A Simulation Study on the iCub Humanoid Robot.” IEEE Robotics and Automation Letters 4.2 (2019): 1900-1907.

[11] Birnholz, Jason C. “The development of human fetal eye movement patterns.” Science 213.4508 (1981): 679-681.

[12] Fagard, Jaqueline, et al. “Fetal origin of sensorimotor behavior.” Frontiers in neurorobotics 12 (2018): 23.

[13] Valman, H. B., and J. F. Pearson. “What the fetus feels.” British medical journal 280.6209 (1980): 233.

[14] Lecanuet, Jean-Pierre, and Carolyn Granier-Deferre. “Speech stimuli in the fetal environment.” Developmental neurocognition: Speech and face processing in the first year of life. Springer, Dordrecht, 1993. 237-248.

[15] Marx, Viola, and Emese Nagy. “Fetal behavioural responses to maternal voice and touch.” PloS one 10.6 (2015): e0129118.

[16] Vaughan, Christopher. How Life Begins. Dell Publishing Company, 1997.

[17] Hoffmann, Matej, et al. “Robotic homunculus: Learning of artificial skin representation in a humanoid robot motivated by primary somatosensory cortex.” IEEE Transactions on Cognitive and Developmental Systems 10.2 (2017): 163-176.


[19] Almli, C. Robert, Robert H. Ball, and Mark E. Wheeler. “Human fetal and neonatal movement patterns: Gender differences and fetal‐to‐neonatal continuity.” Developmental Psychobiology: The Journal of the International Society for Developmental Psychobiology 38.4 (2001): 252-273.


Is it really that dark in the womb? Development of fetus vision.

Have you ever thought about what a baby is actually seeing when it is still in the belly? Or you thought, as well as my daughter, that since the baby is in the dark and in the water, it does not open eyes until it gets out and development of vision just starts at the second when the baby leaves the belly? Yet it seems, that not only that visual system as the whole is developing already since the 4th week of pregnancy (wiring important neural connections, building up tissues, lenses, muscles, and all the other necessary stuff,…), but the baby can actually see inside the womb some light, after 26 weeks it can open eyelids and it keeps eyes open when awake, it reacts to intense light and some recent studies suggest even so crazy ideas such as that babies might recognize faces already in the womb – as it seems that the baby reacts to the light in the shape of face more than to light in different shapes – crazy, right?

Development of the visual system

From the beginning of the 2nd month of pregnancy (at around 4 weeks), the development of the visual system and eyes starts and for next few weeks you can see such a rapid development of all the important parts of the visual system that it seems almost incredible. The fetus is really busy with creating all eye structures and corresponding neural connections. As stated in very nice course [4]: “Eye development during the first trimestr of pregnancy is like watching the grand finale of Fourth of July fireworks. Cells and tissues develop quickly and simultaneously to form various eye structures.” 

At 4 weeks precursor to 2 optic nerves (optic stalk) is created (it is developed at 36 days). Optic nerves create a crucial connection between eyes and forebrain (each one connecting eyes to the one side of the brain) [2, 4]. In parallel, at 4 weeks, first cells start the baseline for lens (which will help the kid to focus on objects), at 32nd day we can already identify them and by the end of the 2nd month of pregnancy they will grow to the size which they will have at birth [4]. Also, at 30-35 days of pregnancy iris (which helps to reduce the amount of light coming to the eye) start to develop and 2 weeks later it is ready. At 4 weeks of pregnancy also extraocular muscles are created, as well as the cornea is developing at this time. The retina (layer in the back of the eye covered by photoreceptors which perceives and processes light and sends signal to the brain) starts its development around 8 weeks of the gestation age.[4]

Eyelid at 8 weeks is still fully closed. Image source: [11]

Eyelids start to form around 8 weeks. They are closed and serve as a protection for other developing eye structures [4]. At 16 weeks, the eye is starting to pick up on light and can perform slight movements from side to side in response to light [2]. By this time, eyelids are still fused and closed. From the beginning of the 5th month till the end of the 6th month, eyelids are slowly separated. At 26 weeks (7 month) of gestation age, highly-hydrated cornea starts to be transparent and enables eyes to sense light. Eyelids finally open. [4] Since then the baby keeps eyelids open when awake. It seems, that the baby is from now on able to sense the movement of bright light outside your body – if you flash bright light to the belly, you might get “kick” response. Between 28 – 30 weeks, rapid eye movements and sleep patterns appear. As will be mentioned later, the REM sleep and sleep patterns are crucial for healthy vision development as they synchronize brain waves of visual system with retina [4].

How dark is it in the womb and how the baby reacts to the light?

As we could see, in the last 2 months of the pregnancy (from the week 30-32), the visual system is already developed enough to be able to react to the incoming light and process some incoming visual information – although with a lower acuity (fetuses would only be able to see large objects or smaller objects close by) and mainly in the grayscale (as the cones are not yet developed). So the question is – how much the baby can actually see in there? Marco del Giudice used measured transmission coefficients of biological tissues and modeled light transmission from the external environment to the uterine cavity. He found out that for the naked woman, around 0.1-1 % of ambient light might get inside the womb. This amount is highly variable and depends on the thickness of the tissue (how fat is the belly) and the strength of the external illumination [7, 8].

Now the question is, how much light is needed for prenatal visual experience? Glass [9] review of studies with newborns (term and premature) suggest that the amount of light needed for fetal vision is way lower than the one of mature individuals. [7] makes a conservative estimate of the amount of light permitting vision of fetus to 10 lx (this is light which an adult needs to read a text). Study published by [7] shows that we can expect that fetus in the womb receives at least 10-100 lx, therefore its visual experience is enabled.

In the recent study (2017) [10] V. Reid et al. used a 4D scanner to explore how fetus reacts to the light source in different shapes. On top, the response of the fetus was recorded when the light source was moving. The visual stimuli was projected through the uterine wall and fetal head turns were examined. They found out that fetus in the third trimester looks toward three dots which are configured upright like a face significantly more than to three inverted configuration dots (the stimuli they used is shown on the figure – and as you can see, interpreting it as face is somehow not that intuitive…at least not for me 🙂 ). 

Visual stimuli used in the study by V. Reid et al. [10]

So it seems that the womb is not that dark place after all…

What can we do during pregnancy for the vision of our baby?

(…how we can help to maintain healthy development of eyes and vision of our precious one?)

It seems that the most important for visual system development is genetic coding [1] (you cannot do much about this part :)), certain nutrients – particularly vitamin A [1,2,5](you can find it in fruits, dairy products and mainly in leafy vegetables), and keeping REM sleep and sleep cycles of the baby undisturbed. Why is REM sleep and sleep cycles so important for the visual system development? During its life in the womb, the brain of the fetus creates topographic relationships between retina, lateral geniculate nucleus and primary visual cortex. To create those mappings correctly, REM sleep (accompanied by rapid eyes movement) and sleep cycles seem to be crucial and sleep disturbance or deprivation might result in the fact that these important bindings are not created properly [1]. 

So maybe we should not try too often to make the baby move by pushing, flashing light to the belly, or impose it to loud noises (you shouldn’t anyway)…

Is it good to stimulate a fetus by light before birth or not?

There is no doubt that simulation of the vision after birth is very important for newborn visual system development [1].  On the other hand, there is contradictory evidence whether simulation of visual system before birth might be beneficial or not. [1] states that it is not important and the visual system cannot benefit from it. On the contrary, there is a study done on pregnant mice [6], which were kept in complete darkness and after birth, their babies were more likely to have vision problems. This study states that fetal mice require light exposure in utero during early gestation for normal vascular development in the eye. In parallel, there were done studies on the pregnant women who live at northern latitudes with pregnancy lasting during the darkest months of the year. Surprisingly, their babies have actually increased risk of certain eye disorders [2,6]. These studies suggest that stimulation of the visual system is important – at least to some extent. Anyway, it doesn’t mean that you should be flashing light to your belly to keep your baby stimulated. Just do not stay inside a cave all the time :).

Note: Keep in mind that pupils start to respond to light earliest in the 8th month and it takes another 2 weeks until the response is consistent. That is why premature babies (born before 34 weeks of gestation) need eye protection to let the eye finish its full development. The need to protect the underdeveloped visual system from light outweighs the benefits of its stimulation until the premature infant is reaching the age typicall for leaving the womb – 40 weeks of gestation. [4]

Can a baby in the womb cry?

Lacrimal glands (where tears are produced) begin to form already during the 6th week of pregnancy. However, they do not produce no tears until the third month after birth, which is why infants shed no tears when they cry. [4] 

Does a fetus/newborn see in colors?

It is important to notice that the retina development goes on from 8 weeks of gestation age till 3-4 months after birth. So at the time when the baby first comes to the world, the optic nerve is still not fully myelinated and rods (photo receptors responsible for grey-scale vision enabling us to see in the dark) are way better developed than cones (enabling color vision). Therefore, newborn cannot see in colors, but mainly perceives shades of gray and discriminates between dark and light. The color vision (enabled by red, green and blue cones) will come later – around 3 months of age – together with maturation of cones [4]. Maturation of the whole visual system and corresponding neural areas will still go on…

Read more…

[1] Graven, 2008: Visual Development in the Human Fetus, Infant,and Young Child




[5] See, Angela Wai-Man, and Margaret Clagett-Dame. “The temporal requirement for vitamin A in the developing eye: mechanism of action in optic fissure closure and new roles for the vitamin in regulating cell proliferation and adhesion in the embryonic retina.” Developmental biology 325.1 (2009): 94-105. 

[6] Yang, Michael B., et al. “Length of day during early gestation as a predictor of risk for severe retinopathy of prematurity.” Ophthalmology 120.12 (2013): 2706-2713. 

[7] Del Giudice, Marco. “Alone in the dark? Modeling the conditions for visual experience in human fetuses.” Developmental psychobiology 53.2 (2011): 214-219. 


[9] Glass,   P. (2002).   Development of   the visual system   and implications for intervention.  Infants and Young Children, 15, 1 – 10

[10] Reid, Vincent M., et al. “The human fetus preferentially engages with face-like visual stimuli.” Current Biology 27.12 (2017): 1825-1828.


Are babies listening to us already in the womb?

Language development is a topic that fascinates me for years and it all started by observing my oldest daughter slowly responding and developing her linguistic skills by 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, which response to touches and voices, I wonder how much it actually hears and how much what it hears affects its future development…

Most of the people and research focuses on the brain of the newborn and the language acquisition after the birth, but brain areas corresponding to the language development appear in the brain already during the 30th week of pregnancy, and as soon as from the 18th week of pregnancy, it can ‘hear’ sounds of your body such as your heartbeat and since the 27th week auditory learning becomes possible…There are studies that show that a fetus in the womb reacts to the mother’s voice, to the music, etc. There are also studies that show that when the baby is exposed to one language during pregnancy and adopted at birth, it can afterward still produce mother language sounds or distinguish some words from that language. All these findings are impressive and bring up an interesting question – how is the prenatal exposure to sounds and languages affecting the later development of the language for the baby? 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 see a bit more, what is happening in the womb, we can explore the life in the darkness and see how the small brain is a step by step developing and responding to the outer world…

Neurophysiology of the fetus

First, let’s see, what happens with the baby brain which enables it to hear and recognize the language, all of which later leads to the fact that newborns might be soothed by familiar sounds like an adult heartbeat or that they prefer mother’s language to other languages…. The onset of hearing in humans is around 27 weeks of gestational age [1], but already since 18th or 19th week, the baby can respond to sounds of the body such as heartbeat or stomach rumbling [2] as the human cochlea starts to be slowly functioning [3]. That is the time when external auditory inputs start to have an effect on the reorganization of the auditory cortex [4]. To test the onset of hearing, [1] performed an experiment, where they exposed the fetus to pure auditory stimuli of different frequencies (100, 250, 500, 1000, and 3000 Hz) – from 19 to 35 weeks of gestational age (GA). Measuring the responses by loudspeaker on maternal abdomen and by ultrasound, they observed first responses for 500 Hz at 19 weeks of GA, then responses to lower frequencies appeared (At 27 weeks, 96% fetuses were responding to 250 and 500 Hz) and the latest ones were responses to higher frequencies (responses to 1000 and 3000 Hz haven’t been observed till 33 and 35 weeks, respectively). As the baby brain matures, a big decrease (20-30 dB) in the intensity level required to elicit a response was observed. This corresponds to the neurophysiological maturation of the inner ear, which seems to be matured at the end of the 8th month when all relevant neuronal synapses take their place [3].

In the 30th week, brain areas corresponding to language development are developing ([5, 6]). As you maybe know, there is observed a left hemisphere dominance for language processing, which seems to be genetically given and we can observe asymmetric development of the brain already before birth. For example, the temporal plane layer in the left hemisphere is larger than in the right one. We can also observe that the left hemisphere has different sensitiveness for speech already from birth (response was measured by electrical activity – event-related potentials, Mehler). Interestingly, the development of cortical regions that are connected to language is slower in the left hemisphere than in the right one (e.g., right temporal plane develops at the 30th week of GA, but left one 7-10 days later) ([7, 5]). The fact that brain regions for language development are distinct already at birth might be a partial verification for Chomsky [8] hypothesis, that there exists a universal innate grammar, which we just fill in with vocabulary and rules.

This above-mentioned development leads to the fact that after 32 weeks of gestation age, the baby may start recognize vowel sounds from your language [2], which further leads to the findings which we will mention later, that the newborn prefers the native language to other languages [9].

Sound and music in the womb – is the baby partying there?

How can we measure what the baby actually hears? It is easy. 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, body noise and mainly low frequencies around 500-700 Hz, all of which are highly attenuated [10, 11, 12]. Nevertheless, mother voice and external speech will still be clearly emerging above the background noise with well preserved prosodic characteristics. [11, 12] found out that even some phonemes/words are still recognizable (up to 30%). All these speech connected sounds are quite silent as they are attenuated significantly. Although you can hear all (close by) external speech, you will get during your life in the womb most familiar with the mother’s voice, because there is significantly better transmission of maternal voice than of external voices – Querleu found out that there is 20dB attenuation of external voices and only 8 dB of mother voices [11]. How can this be explained? It seems that there are 2 pathways transmitting the voice – one is the same for both maternal and external speech, but the mother’s voice is transmitted also through body tissues and bones which enables kids to get more used to this voice.

 This is closely connected to the findings on the newborns, that mother’s voice has calming effects for < 2-week old kids and is more attractive for the newborn than other female voices. 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. [10a] Spence [13] went even further and found out that infants prefer the maternal low-pass filtered voice, but not the maternal whispered voice.

What is another interesting finding is that external auditory stimuli in general increases motor responses (both long term and startle responses) as well as heart rate responses [10], but the mother speech can decrease fetal heart rate – the most when the speech is in low frequencies and around 70dB. The responses of the fetus were measured for tones, vibratory frequency noises, or EAL stimulations during quiet and active sleep and during active and quiet wakefulness of the baby – it has been found that although to tones the baby starts to respond at 27-28th week of GA, to EAL stimuli motor responses appear already in the 24th week [10]. Bigger responses can be observed during wakefulness than during sleep. Unborn babies might even start to cry if exposed to a sudden loud noise.

And how is it with the music? Feijoo [14] 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 last 3 months of the pregnancy way more than any other music.

Postnatal effects of prenatal stimuli

So you can see that it has been found out, that familiar sounds might soothe crying newborn [10], including adult heartbeat, mother’s voice[13, 10], music [14], or soap opera theme [15].

Question is, how can we find out what a newborn actually prefers? There is an experiment called the non-nutritive sucking behavior of the newborn which is supposed to measure babies’ responses to individual stimuli. What should this mean? It is easy, this technical term hides an easy thing – behavior while sucking on a pacifier. So basically we measure how much the baby is sucking, what are the pauses between sucking and frequency of sucking. So easy :).

Using this method, it has been found that newborn babies exposed to the adult heartbeat (72 beats/s) were sleeping earlier and gained weight quicker [16]. Also, intrauterine background noise had a short term pacifying effect ([10a], Murooka [17]). Familiar stimuli were also preferred by a child in selection tasks.

Back to mother’s voice…DeCasper [18] 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. And have you ever heard about fetal ‘soap’ addiction? It was described by Hepper in 1988 [15], [19]. What they found out? That there were 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 these 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.

So far I was mainly speaking about voice and sounds, but what about the actual languages? Moon [9a], measured 40 infants about 30 hours old in the nursery using the test for sucking behavior when listening to vowels from native and unfamiliar language. They found out that babies were sucking longer for the foreign language than for the native tongue (independent on time since birth, 7-75h), which indicates that they can differentiate between these two languages and it seems that the ambient language to which fetuses are exposed in the womb affects their perception of the native language at phonetic level. In another study, Moon et al. [20] measured 2 day old kids of French and Russian mothers while the same bilingual woman was talking to them. These kids again preferred the mother’s tongue to the unfamiliar one and the same behavior was observed also for low-pass filtered versions, which kept only the prosodic cues of the languages. On top when they measured kids from families speaking neither French nor Russian, they were not able to discriminate between these 2 languages. [21]

Maybe you might be a bit skeptical about this sucking behavior test…Then there is another study conducted by May et al. [22], who measured a neural activity of the brain using NIRS (near-infrared spectroscopy) study on 20 monolingual English 0-3 days old neonates. She let them be exposed to low-pass filtered sentences of forward English (familiar language), forward Tagalog (unfamiliar language), and backward English and Tagalog (non-language) and measured neural activation by NIRS. They found out bilateral (in both hemispheres) activation connected to the familiarity of the language and observed different activation for Tagalog (unfamiliar language). This indicates that exposure to the language in uterus influences the brain responses of the baby to the native language after birth.

So it actually seems that babies in the womb really listen to us as soon as their hearing enables them so, although what comes to them is highly attenuated and filtered for high frequencies. After several months of this auditory experience, they become attracted to the familiar sounds and patterns of the native language(s). But it is possible that they actually learn even more from their prenatal experience as they might become familiar with the prosody of the languages heard in utero (‘prenatal prosodic bootstrapping’), which might be crucial for later language acquisition [23]. To conclude, we can see that long-term premature exposure to auditory (or other sensory stimuli) in utero might lead to both prenatal and postnatal responses to these stimuli.

And one last thing to mention, the auditory and language system as a whole continues, of course, to develop also after a birth – e.g., hearing develops for another 5-6 months after 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 sensible 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…


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