Women who are stressed in pregnancy may raise the risk of their child developing asthma or other allergies, a study suggests.
Researchers found higher levels of a chemical linked to allergy in the blood of children of stressed mothers.
Levels were high even in those who had not been exposed to high levels of dust mites, a recognised allergies trigger.
Harvard Medical School, which studied 387 babies, will present the study to the American Thoracic Society.
It is thought the risk of asthma and allergy is controlled by a complex mix of genetic and environmental factors.
However, researchers 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.
The Harvard team examined the theory that stress during pregnancy can magnify the effect of foetal exposure to substances which can trigger allergy.
The researchers measured levels of Immunoglobulin (IgE) - a chemical linked to allergic responses - in the umbilical cord blood of 387 babies.
Because even babies exposed to low levels of dust mites in the uterus showed elevated levels of IgE, the researchers concluded that stress was amplifying their allergic response.
Researcher Dr Rosalind Wright said: "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.
"This further supports the notion that stress can be thought of as a social pollutant that, when 'breathed' into the body, may influence the body's immune response similar to the effects of physical pollutants like allergens, thus adding to their effects."
However, Dr Wright said more work was needed to tease out further the effect of stress from other possible factors which may influence allergy risk.
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.
"These preliminary findings are of interest and potential importance and further support the need for a healthy and balanced lifestyle during pregnancy, as it could lead to a reduction in asthma and allergies."