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The role of stress in asthma is not understood, but animal studies suggest that prenatal stress can influence the infant's immune system in the womb, the researchers noted. It is also known that asthma is most prevalent in inner cities, where minorities and disadvantaged people live in increasingly stressful circumstances, they added.
"This is the first human study to corroborate research from animal studies demonstrating that stress experienced by mothers during their pregnancy influences their child's developing immune system starting in the womb," said lead researcher Dr. Rosalind Wright, an associate physician at Brigham and Women's Hospital and an assistant professor of medicine at Harvard Medical School in Boston.
"The work may point to the need to design interventions and strategies to reduce stress in pregnant women to both enhance the mother's well-being and to reduce the risk of chronic illnesses in their children such as asthma," she noted.
The report is published in the March 18 online edition of the American Journal of Respiratory and Critical Care Medicine.
For the study, Wright's team surveyed pregnant women in several cities, including Boston, Baltimore, New York and St. Louis. The women were mostly from ethnic minorities, and 20 percent lived below the poverty level.
In each of the 557 families, a mother or a father had a history of asthma or allergy.
All of the families completed a questionnaire that asked about the stress they lived with, such as domestic violence, money worries and violence in the community.
After the babies were born, Wright's group took samples of the umbilical cord blood. They used these samples to test reactions to various allergens, such as dust and cockroaches, and viral and bacterial stimulants.
Children born to more stressed-out moms-to-be had different immune cell responses when stimulated with various common environmental triggers compared to babies born to mothers reporting less stress, Wright said.
The researchers were particularly interested in the production of cytokines, which are proteins released by immune-system cells that help govern immune responses.
"The cytokine patterns seen in the higher-stress groups, an indicator of how the child's immune system is functioning at birth and responding to the environment, may be a marker of increased risk for developing asthma as they get older," Wright said.
Wright's team plans to follow the children as they grow up to see if they are at increased risk for developing asthma or allergies.
Dr. Steven E. Lipshultz, chairman of pediatrics at the University of Miami Miller School of Medicine, said these findings aren't surprising, but whether they predict a child's risk of asthma or other allergies is unclear.
"This is an intriguing finding," Lipshultz said. "It isn't surprising, preclinical studies have supported this, what's novel here is it's the first human study."
Lipshultz noted that during the first few months of life the infant is protected by the mother's immune system. "Then the baby has to start making their own immune system responses," he explained.
That's why it's tough to take these findings to the next stage, Lipshultz said: "This study can't tell you, based on whether a mom is stressed during pregnancy, that the child will be healthier or sicker from asthma or allergic diseases." He said the only way to tell is to follow these children as they grow up.
Another expert, Dr. Andrew R. Colin, director of the division of pediatric pulmonary and co-director of the Pediatric Cystic Fibrosis Program at the University of Miami Miller School of Medicine, said stress is a well-known risk factor for asthma.
"But this is a completely different understanding of the etiology [of asthma], that this is transmitted from generation to generation -- this is really interesting," he said.
"That the immunity of the mother is actually modified by stresses in the environment and transmits [it] to the next generation is not new. There is data that it goes even to the third generation," he said. "But the fact that stress is the major player in being the immune modulator, I think is really exciting."
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