Children whose mothers were chronically stressed when they were young infants are more likely to develop asthma than their peers, research shows.
Lead study author Anita Kozyrskyj said: "It is increasingly clear that traditional environmental risk factors do not fully explain the origins of asthma.
"Evidence is emerging that exposure to maternal distress in early life plays a causal role in the development of childhood asthma."
A study of 14,000 children in the US found that maternal stress increased the prevalence of childhood asthma by nearly a third.
The risk of developing asthma increased when children lived in high-income households or had more than one sibling.
Author Kozyrskyj added: "We plan to further explore the role of postpartum distress by doing a similar study which will link health care records with public health nurse assessments of depression and anxiety from a provincial postnatal screening program. This will enable us to assess the effects of less severe depression and anxiety during the postpartum period."
The findings were published in the American Journal or Respiratory and Critical Care Medicine.