In The Womb | Begin Before Birth

Fetal Programming

The fetus in the womb

Fetal development is a very complex process. At different stages of development different aspects can be changed by specific outside influences.

This means that external factors can actually affect the development of the fetus, and some of these changes can last for life.

Some chemicals that can harm fetal development are well known and include:

  • Too much alcohol during pregnancy can lead to fetal alcohol syndrome. Even small amounts of alcohol can have an effect on brain development.
  • Smoking during pregnancy can slow the growth of the fetus and also affect some aspects of the development of the brain.
  • The drug thalidomide can disrupt the development of the limbs leading to physical deformity and disability.

Fetal programming is the idea that:

The environment in the womb, during different sensitive periods for specific outcomes, can alter the development of the fetus, with a permanent effect on the child.

Fetal programming can be altered by quite subtle changes due, for example to altered nutrition, or maternal stress. The effects of these changes do not always become obvious at once, but sometimes only show up later on.

In the first few weeks the physical structures of the baby in the womb are being formed.  That is why the drug thalidomide had an effect on the development of arms and legs, only if it was taken early in pregnancy. But the brain is being formed all through pregnancy and its development can be affected even at later stages.

How the baby grows in the womb is important too. Growing more slowly doesn’t just affect early life. It can impact health throughout life, even in old age. This is because some of the organs such as the kidney and the pancreas which have also grown differently at the beginning, are less able to function properly in old age.

These two babies were both born full term from healthy pregnancies, but one has a much lower birthweight than the other.

Birth weight can reflect life in the womb

A scientist called David Barker, in Southampton, England found that the lower the birthweight, the higher the risk of dying from heart disease in later life.

This led David Barker to make the following hypothesis:

“Coronary heart disease, type 2 diabetes, stroke and hypertension originate in developmental plasticity, in response to undernutrition during fetal life”.

There are many causes for babies growing more slowly in the womb, the mother’s nutrition is only one of many.  Being smaller at birth is only a risk factor for these later problems. Many low birthweight babies never have them.

The Barker hypothesis has now been proved by many studies around the world. Now scientists are trying to  understand  what factors affect fetal development, and health and wellbeing throughout life.


If the mother experiences emotional stress during pregnancy this can also affect the development of the fetus, especially of the brain. This effect can occur at any stage during pregnancy. Early in pregnancy the nerve cells move to their final position, and later they make contact with each other and link up in specific pathways.

Fetal brain development

These images illustrate fetal brain development.

The brain increases rapidly in size in the later stages of pregnancy. The brain gradually develops more folds as the amount of brain tissue increases.

During the last 20 weeks of pregnancy, the brain increases in size 17-fold.

This is when the nerve cells called neurones are starting to join up with each other, and basic functions start to become ‘hard-wired’ in the brain.

Higher levels of the stress hormone cortisol have been shown to change the development of the fetal brain.

The impact of prenatal stress on the fetal brain does not occur in every case. Some fetuses  are much more affected than others.


Fetal Development

It takes about 38 weeks for a fertilised egg to develop into a full term baby.

The changes that occur over this short time are quite amazing.

This page describes some of the things that a fetus can do, and when they start (see here for an animation on fetal development).

All of the fetus’s senses will be stimulated naturally during the course of pregnancy, except for vision.  The fetus has the ability to detect stimuli from as early as 8 weeks, in the case of touch.  The environment of the fetus is not one of sensory deprivation.

The older fetus learns, while in the womb, to recognize certain sounds or tastes, and this prepares them for life after birth.

Note: Pregnancy is usually measured from the date of the beginning of the last  period, and the egg is fertilised about 2 weeks after that.  So although pregnancy is said to last for 40 weeks gestation, for the first 2 weeks of the 40 weeks of pregnancy there is nothing there.

Fetal Movement

A 3 month old fetus

The human fetus at 3 months

Ultrasound technology enables us to clearly observe the fetus in the womb

Pregnant women usually first feel their baby move between 16-20 weeks gestation

The first movements are seen on ultrasound at 8 weeks gestation

The fetus can suck his/her thumb from 20 weeks

Common patterns of behaviour seen in adults are also seen in the fetus – e.g. handedness. Fetuses that suck their right thumb rather than their left thumb are likely to be right handed children.

Fetal movements increase and become more complex as the pregnancy progresses.

It is important for the fetus to practice all the movements that will be needed at birth for two reasons:

  • Firstly it helps the brain and nervous system to develop well
  • Secondly it allows the joints, bones and muscles to form correctly

Fetal breathing

The fetus does not actually breathe in the womb. The mother breathes for the fetus, and essential oxygen is passed to the fetus through the umbilical cord.

The fetus does make breathing-like movements though. These begin at 9 weeks of pregnancy and allow the fetus to practice this breathing movement.

This means that when the baby is born, he/she will be able to breathe straight away.

Eye movements

The fetus begins to move his/her eyes during the 14th week of pregnancy.

Complex movements, similar to our eye movements are present by the 24th week.

In the last third of pregnancy, rapid eye movements have been seen in the fetus. In adults, these occur when we are dreaming. Perhaps the fetus is dreaming too.

Recognizing sounds

The fetus begins to respond to sounds at about 20 weeks into the pregnancy. To begin with the fetus only hears low noises, but as development continues he/she starts to hear higher pitched noises too.

Louder sounds can make the fetus startle and move about..

The older fetus is able to discriminate between different voices, languages, and even individual speech sounds, e.g. “BABI” and “BIBA”.

A newborn can recognise music that he or she heard in the womb.

Urinating, Drinking and Tasting

The fetus starts to empty his/her bladder during the 10th week of pregnancy. Urine is passed straight into the amniotic fluid, the protective fluid surrounding the fetus in the womb.

About five weeks later, the fetus starts to drink this same fluid.

So, while in the womb, the fetus is exposed to different tastes through the amniotic fluid.

The fetus likes certain tastes more than others, and will drink more amniotic fluid if it tastes sweet.

Becoming conscious/Feeling pain.

People often wonder when the fetus starts to be aware of itself or of signals coming in from the outside world.

When does he or she start to hear sounds or feel pain? We know with each other when someone feels pain because we can ask them and they can tell us. We cannot do this with the fetus, so we have to study when the brain develops enough to make this possible, and make an informed guess.

It is not enough just to study physical responses. For example, if you touch the foot of a fetus at 12 weeks, he/she will move away, but it is extremely unlikely that the fetus is feeling anything. This is just a reflex response like a knee jerk.

At this stage there is no connection between the nervous system in the body and the higher levels of the brain, which we think  are necessary for  conscious experience.

These connections between brain and body, necessary for feeling pain or hearing sounds, do not begin to start until about 17 weeks at the earliest, and are in general well formed and functional by about 26 weeks.

From 26 weeks onwards it does seem likely that the fetus can hear sounds and feel pain too.  From about 20 weeks they may hear or feel something but it is hard to be sure.

Prenatal stress- fetal responses

The older fetus can sense what the mother is feeling. If the mother feels stressed or anxious playing a computer game, scientists have shown that the fetal heart rate goes up while she is playing it (see Monk and colleagues, 2000 or click here for more information).

Try our Stroop game to see this .