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.
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.
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.