Researchers have warned that stress during pregnancy can have an adverse impact on offspring, leading to slow development, learning and attention difficulties.

It can also cause developmental and emotional problems for offspring, they add.

In a study on rats, a research team led by Marta Weinstock-Rosin of the Hebrew University of Jerusalem School of Pharmacy compared the behaviour of the offspring of stressed rat mothers with those whose mothers were not stressed.

The researchers found that offspring of rat mothers subject to stressful situations such as irritating sounds at alternating times later shown to have impaired learning and memory abilities, less capacity to cope with situations like food deprivation, and symptoms of anxiety and depressive-like behaviour, as compared to those rats in control groups that were born to unstressed mothers.

Further analysis also showed that adverse impact of excessive levels of the hormone cortisol, which is released by the adrenal gland during stress and reaches the foetal brain during critical stages of brain development.

The team found that under normal conditions, this hormone has a beneficial function in supplying instant energy, but it has to be in small amounts and for a short period of time.

Under conditions of excessive stress, however, the large amount of this hormone reaching the foetal brain can cause structural and functional changes.

In humans, above-normal levels of cortisol can also stimulate the release of another hormone from the placenta that will cause premature birth, another factor that can affect normal development.