Pam Versfeld   August 2021

In this post I reproduce (with some editing, bolding and changes in layout) the comprehensive description of fetal movements provided in the article Fetal Origin of Sensorimotor Behaviour. For references please see the original article. 

Fagard, J., Esseily, R., Jacquey, L., O'Regan, K., & Somogyi, E. (2018). Fetal Origin of Sensorimotor Behavior. Frontiers in neurorobotics, 12, 23. https://www.frontiersin.org/articles/10.3389/fnbot.2018.00023/full

In this article Jaqueline Fagard and co-authors make the case for considering development as the self-organizing emergence of complex forms from spontaneously generated activity, governed by the innate capacity to detect and memorize the consequences of spontaneous activity (contingencies), and constrained by the sensory and motor maturation of the body.

In support of this view, the authors show how observation of  fetuses and also several fetal experiments suggest that the “fetus’ first motor activity allows it to feel the space around it and to feel its body and the consequences of its movements on its body. This primitive motor babbling gives way progressively to sensorimotor behavior which already possesses most of the characteristics of infants’s later behavior: repetition of actions leading to sensations, intentionality, some motor control and oriented reactions to sensory stimulation.”

In this way the fetus can start developing a body map and acquiring knowledge of its limited physical and social environment.

The first weeks of gestation: general movements and startles

Motor patterns most characteristic of the first weeks of gestation are spontaneous startles, general movements (GMs), isolated movements and twitches. In general, the number of fetal movements per hour increases until a plateau is reached and decreases from 16 weeks onward

General Movements are characterized by their fluency and their variety, and their repertoire increases rapidly from 8 weeks to 10 weeks.

At the beginning, startles are often followed by GMs, in which all parts of the body participate.

After the 17th week, GMs do not necessarily follow a startle and appear spontaneously.

Isolated movements, which emerge soon after GMs involve distinctive sequencing of particular body parts.

The onset of isolated movements is simultaneous for arms and legs  but arm movements are more frequent than leg movements, at least in 14- to 18-week fetuses.

Twitches are a particular kind of spontaneous motor activity produced during active sleep. Brief contractions of muscles trigger quick extensions or flexions of a limb or the neck.

Fetuses start producing twitches at the age of 10–12 weeks, and from 15–16 weeks the frequency of twitches increases substantially. Even though they appear during sleep twitches may have an important role for establishing the brain’s body map.

Emergence of sensory experience and sensorimotor behavior

Even though early motility appears to be mostly unrelated to sensations, it is difficult to determine precisely when a fetal movement is spontaneously initiated or when it is triggered by sensations, due to movements of the mother or to internal sensations.

Reflexive reactions to touch occur almost as early as spontaneous motor behavior.

Early instances of reaction to touch can be observed in the case of twins: twins react strongly when they are touched or pushed by the other twin. Such reactions can be observed between 11 and 13 weeks—earlier in the case of monochorionic twins.

As the sensory systems develop, non-reflexive responses to stimulations can be observed.

The fetus’ environment is often disturbed by sounds, light, and touch and the fetus soon responds to these disturbances by moving.  

21-37-week old  fetuses respond to maternal touch of the abdomen or to vibroacoustic probes by an increase of arm, head and mou

Motor Babbling (GMs and Isolated Movements)

The first movements of the fetus, general or isolated, give the impression, not only of being spontaneous and not in reaction to sensation, but also of not being aimed at a precise goal, but rather to be randomly distributed across the space around it.

Motor babbling is the term used for these seemingly random movements.

"Even in the apparent absence of an intentional goal, motor babbling allows the fetus to explore the space around it, to explore its body and its environment and to explore the consequences of its movements on its body and on its environment."

“One could compare fetuses to astronauts exploring space, driven by some kind of primitive curiosity or intrinsic motivation. Intrinsic motivation refers to living creatures’ (especially young ones’) search for novelty, in other words to a behavior that does not lead to the satisfaction of physiological needs but rather to an increase of knowledge (about own body and environment).”

Spontaneous motoneuron activity begins at the same time as motoneuron differentiation. Indeed, motor activity starts as rhythmic bursts of spontaneously generated action potentials correlated across thousands of cells, at a stage when motor neurons are pathfinding and innervating the skeletal muscles.

Early fetal movements are canalized by some constraints, arising from the system itself (characteristics of the articulations, state of development of the nervous system) as well as by the characteristics of the environment.

These characteristics change through pregnancy, and due to these changes the contingent effect of the same movements may change. Due to the aquatic environment, arm and leg movements are likely to turn the body around as long as there is enough space and enough amniotic fluid around the fetus.

By the end of pregnancy, when space is shrinking as the fetus grows, most arm movements end up not far from the face.

The nervous system also is changing.

"From the beginning, there are two cortico-spinal tracks, one descending directly toward the spinal cord and the periphery (ipsilateral), the other one crossing the corpus callosum and descending on the opposite side of the spinal cord and periphery (contralateral). At first control is ipsilateral but becomes increasingly contralateral as the corpus callosum develops. The decrease in the amount of movement during pregnancy is believed to be due, not only to the restriction in available space, but also to the emergence of inhibitory cortical influences."

"Within these constraints, motor babbling is extremely variable within and across fetuses. It may result in accidental contacts with the body or with the uterine environment. Such accidental contacts appear to be held in a memory of consequences, in such a way that the fetus soon starts to show a repertoire of “preferred” movements."

From Motor Babbling to Sensorimotor Maps

To repeat a movement, the fetus must know the connections between motoneurons and muscles, in other words it must have some sort of sensorimotor mapping.

Scientists increasingly believe that sensorimotor mapping emerges progressively from spontaneous movements. Indeed, there are no movements without sensory consequences (the reverse being not true since sensory stimulations are not always followed by movements).

Even twitches, produced on a background of muscle atonia (during sleep), are believed to play a fundamental role in the self-organization of spinal and supraspinal sensorimotor circuits and body mapping.

“GMs are like sensorimotor “storms” during which tactile, proprioceptive and vestibular sensations are simultaneously elicited (Piontelli, 2010).

Isolated movements allow the fetus to touch different parts of the body, with the back or with the palm of the hand, or with the foot. Touching induces double tactile stimulation: stimulation of the active touching part (hand, leg, tongue) and passive stimulation of the touched part.

Therefore, even before the brain starts to receive significant sensory input from the outside world, spontaneous movements provide sensory stimulations. As a result, GMs and isolated movements are important not only, as all other spontaneous movements, for the development of the motor machinery of muscles, tendons, ligaments, cartilages, spindles and bones  but also for the development of sensorimotor circuits and sensorimotor mapping.”

"Thus, fetal sensory stimulations arise from several sources, from endogenously-triggered spontaneous movements as well as from other sources, to stimulation arising from fetal or outside environment. All are likely to contribute to the development of somatosensory cortex and to the formation of cortical body maps."

Early goal-directed movements are channelled by sensory consequences

Fetuses soon seem to increasingly prefer moving parts of the body that are richly innervated.

• Starting at 10–12 weeks, face contacts are seen very often.
• Arm movements aim increasingly toward the mouth as pregnancy progresses. The mouth and the thumb are both highly innervated and perhaps this is the reason why fetuses seem to like thumb sucking.
• Other self-touch behaviors observed in utero include rubbing the eyelids, scratching the temples with the fingers, which, even without nails, may elicit sensations.
• The cranium, which is very little innervated by sensory fibers, is rarely scratched, except the part that is more innervated such as the occiput and the nape.

After the first eye motions, at 16–18 weeks, the fetus starts touching its eyelids, closed until 23–24 weeks. Retina development is well advanced at that age and rubbing the eyelids may generate flashes of light in the fetus.

Fetuses also touch their feet, in particular the soles that are well innervated.

Fetuses rarely touch other parts of their body that are less sensitive, like the abdomen or the thorax.

Movements in the third trimester

The most frequent movements in the third trimester are facial movements (eyelids and mouthing movements; hand to face, hand to eye, hand to head, scowling, eye and mouth opening,

At first, hands move independently. At 20–22 weeks, fetuses can be seen touching one hand with the other or crossing hands. They may also grasp the umbilical cord when they accidently contact it, thanks to the grasping reflex. 

If the fetus increasingly aims its movements toward the more sensitive body parts, this means that it progressively selects these movements that induce interesting sensory feedback.

First steps towards action planning 

Fetuses start to anticipate the consequences of their movements, which may be a first step toward action planning. Studies have shown that:

• Fetuses anticipate their movement toward the mouth by opening the mouth before the hand arrives. This anticipation seems to increase in frequency as gestation progresses.”
• Arm movements toward the mouth become more direct from the 22nd week onwards than before.
• The dynamics of arm movements heading toward the mouth is different from the dynamics of the movements toward the eye, for which the arm slows earlier during the movement, reaching the eye more carefully than the mouth.

Development as the self-organizing emergence of complex behaviour

"The fetus’ motility is no longer seen as a purely reflexive behavior, or as simply emerging from motor primitives hardwired in the spinal cord or brainstem. And development itself is no longer considered as the results of increasing cortical control over lower reflexes through an unfolding program."

Rather, development is now considered as the self-organizing emergence of complex forms from the spontaneously generated activity inherent in individuals with a nervous system, from the sensory constraints due, for instance, to the non-uniform distribution of tactile sensors, and from the capacity to detect and memorize the consequences of spontaneous activity

 It is this interaction between genetically-driven spontaneous activity, genetically-driven basic capacities to detect affordances and regularities, and constraints or channeling due to body and environment that explains behavioral development.

Moving and its sensory consequences allows the fetus to pick up information for making sense of itself and the world, in other words allows sampling of itself and of the world through action.

In turn, this continuous sampling of information modifies input statistics. This leads to changes in brain networks, permitting new behaviors.

“As isolated movements change along pregnancy, the fetus’ sensorimotor behavior comes to possess some of the characteristics later observed in the child’s behavior: curiosity or intrinsic motivation to explore surrounding space and the body, detection of contingencies, repetition of actions leading to sensations, reaction to sensory inputs, intentionality, goal-directed movements and some motor control.”

“The difficulty in considering sensorimotor fetal behavior as already possessing the main characteristics as infants’ sensorimotor behavior is that birth may create a discontinuity in motor control and that one has to wait a few weeks to observe the infants displaying a behavior comparable to the fetal behavior"

See also
Video: Understanding sensorimotor development

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Understanding infant learning - a series of tutorials