How, when and where a child is born may all play a role in
lifetime asthma risk, new studies suggest.
Asthma occurs when airways in the lungs spasm and swell,
restricting the supply of oxygen. The incidence of asthma in the
United States has risen steadily for more than two decades, and
about 6 percent of children now have asthma, up from less than 4
percent in 1980, according to the Centers for Disease Control and
Prevention.
The reasons for the increase are not entirely clear. Genetics
probably plays a role in the risk for asthma, but an array of
environmental factors - pollen, dust, animal dander, mold, cockroach
feces, cigarettes, air pollution, viruses and cold air - have all
been implicated in its development.
This month, The American Journal of Respiratory and Critical Care
Medicine is reporting that children born in the fall have a 30
percent higher risk for asthma than those born in other seasons. The
finding is based on a review of birth and medical records of more
than 95,000 children in Tennessee.
A possible explanation is that autumn babies tend to be about 4
months old at the peak of cold and flu season. By that age, many
babies are in day care and regularly exposed to the outside world.
And while their lungs are still developing, they have yet to
develop strong immune systems. As a result, fall babies are at
particular risk to contract a severe winter virus, which may in turn
increase their risk for asthma.
The lead researcher, Dr. Tina Hartert, director of the Center for
Asthma Research and Environmental Health at Vanderbilt University,
says some parents with a high familial risk for asthma may want to
consider timing conception to avoid a fall birth.
But since that is impractical for many people, Hartert says, all
parents should take precautions to reduce a baby's risk of a
respiratory infection.
"It's premature to say you should time conception so children
aren't born in the fall," she said. "But it's good sense to use
typical hygienic measures to try and prevent illness."
As for how a baby is born, Swiss researchers are reporting in the
journal Thorax this month that a Caesarean delivery is linked to a
much higher risk for asthma compared with babies born vaginally.
In a study of nearly 3,000 children, the researchers found that
12 percent had been given a diagnosis of asthma by age 8. In that
group, those born by C-section were nearly 80 percent more likely
than the others to develop asthma. The explanation may be that a
vaginal birth "primes" a baby's immune system by exposing it to
bacteria as it moves through the birth canal.
Finally, researchers at Tufts reported last month in The Journal
of Asthma that a baby's place of birth also influences asthma risk.
In a study of black families in Dorchester, Massachusetts, they
found that babies born in the United States were more likely to have
asthma than black children born outside the country.
The reason for the disparity is not clear, but the sterile
conditions under which American babies are born may be a factor.
Babies in developing countries encounter more infections, so they
may be better equipped to withstand less serious assaults associated
with asthma, like mold and dust mites.
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