Children whose mothers are chronically stressed during their early years have a higher asthma rate than their peers, regardless of their income, gender or other known asthma risk factors.
"It is increasingly clear that traditional environmental risk factors do not fully explain the origins of asthma," said lead investigator, Anita Kozyrskyj, Ph.D., Associate Professor in the Faculty of Pharmacy at the University of Manitoba, Canada. "Evidence is emerging that exposure to maternal distress in early life plays a causal role in the development of childhood asthma. In a cohort of children born in 1995, we found that maternal distress which persists beyond the postpartum period is associated with an increased risk of asthma at school-age."
The findings appeared in the second issue for January of the American Journal or Respiratory and Critical Care Medicine, published by the American Thoracic Society.
Dr. Kozyrskyj and her colleagues analyzed the medical records of nearly 14,000 children born in Manitoba in 1995 who were continuously registered with Manitoba Health Services until 2003. They determined whether the children had current asthma at age seven by analyzing records of doctor visits, hospitalizations and medications in the year of the child's seventh birthday, and related it to maternal distress as defined by doctor visits, hospitalizations and medication for depression and anxiety. Maternal distress was categorized according to onset and duration into four categories: no distress, postpartum distress only, short-term distress and long-term distress.
"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 Dr. Kozyrskyj.
Even after controlling for the known risk factors of male gender, maternal asthma, urban location and total health care visits, long-term maternal stress was associated with an increase of nearly a third in the prevalence childhood asthma. This is the first study of a non-high-risk cohort of children to report an association with childhood asthma.
The study also found that the risk of asthma associated with maternal stress was intensified under specific home environments: children of mothers with chronic distress who were living in high-income households or who had more than one sibling were more at risk of developing asthma than others whose mothers showed distress.
The mechanisms for how maternal distress causes asthma are not well understood. Depressed mothers are more likely to smoke and less likely to breastfeed--actions which are associated with the development of asthma. However, research has also suggested that depressed mothers are also less likely to interact with their infants. Animal studies indicate that decreased attentiveness from the mother affects the infant's stress and immune response, but the same effect in humans has not yet been definitively demonstrated.
"Our maternal distress measure captured women who sought health care for their depression and anxiety, and thus, our findings may be limited to more severe depression and anxiety," said Dr. Kozyrskyj. "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."