US scientists tested umbilical cord blood and found higher levels of immunoglobulin E – a protein linked to allergies – among babies whose mothers were stressed while pregnant.
Mothers were then exposed to varying levels of a dust mite allergen. Stress appeared to magnify the effect of the dust mite on foetuses, and even women with low exposure had babies with raised IgE if they had been stressed.
It is thought the risk of asthma and allergy is controlled by a complex mix of genetic and environmental factors.
However, researchers at Harvard Medical School suspect the impact of these factors may also in some way be influenced by the environment a foetus is exposed to while still in the uterus.
“This research adds to a growing body of evidence that links maternal stress, such as that precipitated by financial problems or relationship issues, to changes in children’s developing immune systems, even during pregnancy,” said Dr Rosalind Wright, of the Harvard Medical School in Boston.
Previous animal experiments have also suggested that a combination of stress and allergy triggers during pregnancy can affect an offspring’s immune system.
Dr Mike Thomas, chief medical adviser for the charity Asthma UK, said: “The link between stress and asthma has long been recognised and, although it’s still not fully understood, part of the link may be due to the effects of stress on the body’s immune system.