Stressful events during pregnancy causes the placenta – which supplies the foetus with nutrients – to emit cortisol.
As a result, a small amount of this hormone also enters the amniotic fluid – a yellowish liquid that surrounds the foetus – and affects foetal metabolism.
"If the mother is stressed for a longer period of time, the cortisol level in the amniotic fluid increases," said Pearl La Marca-Ghaemmaghami, psychologist at the University of Zurich in Switzerland.
This higher concentration of stress hormone in turn accelerates the growth of the foetus.
"An excessive acceleration of growth may occur at the expense of the proper maturation of the organs," added Ulrike Ehlert, psychologist at the University of Zurich.
If an expectant mother is severely stressed over a longer period of time, the risk of the unborn child developing a mental or physical illness later in life – such as ADHD or cardiovascular disease – increases, the researchers noted, in the paper detailed in the journal Stress.
However, short-term stress situations, did not seem to have an unfavourable effect on the development of the foetus.
For the study, the team tested 34 healthy pregnant women, whose cortisol levels in the saliva were compared with the cortisol level in the amniotic fluid.
Pregnant women who are exposed to longer-term stress situations need to "seek support from a therapist to handle the stress better," the researchers suggested.