Children whose mothers are chronically stressed in the early years of their life are much more likely to develop asthma, suggests new Canadian research.

When University of Manitoba researchers pored over the medical records of 13,907 children born in Manitoba in 1995 and tracked until 2003, they found the duration of mental distress in the mother influenced the development of asthma in their kids.

Just under 19 per cent of the children surveyed were exposed to maternal distress during the first year of life. Of these kids, 8.3 per cent had developed asthma at seven years of age, compared with 6.3 per cent of kids whose mothers weren't stressed.

Long-term maternal distress, on the other hand, was associated with 1.6 times the increased risk of asthma at seven years.

Mental distress was assessed by the number of doctor's visits, hospitalizations and medications the mother took for mental issues such as anxiety and depression.

Researchers also found that kids with highly stressed mothers who lived in high-income homes or who had more than one sibling were at an even higher risk of becoming asthmatic than children in lower-income, one-sibling households.

Children with moms who were stressed or anxious for only the first year of life showed a much-reduced risk.

"Unlike existing studies that have measured maternal stress during the first few years only, the longitudinal nature of our health care study enabled us to characterize maternal distress over time to identify whether it continued," said Anita Kozyrskyj, associate professor in the Faculty of Pharmacy at the University of Manitoba, in a release.

Researchers believe that an increase in women's stress levels overall plays a key part in the onset of asthma. "Growth in chronic stress of women has paralleled the rising prevalence in asthma in the Western world," reads the study. "Stress is well-known precipitant for asthma exacerbations in children.

They believe that stress responses of the mother may create an exaggerated stress response in the baby, which then leads his or her immune system to over-react. Asthma is a disease in which the lungs become oversensitized to stimuli.

Researchers also theorize that stressed mothers don't bond as effectively with their babies. "Depressed mothers demonstrate less attention and fewer responses to infant cues," reads the study.

The study is published in the American Journal of Respiratory and Critical Care Medicine.