Asthma occurs when airways in the lungs spasm and swell, restricting the supply of oxygen. The incidence of asthma 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 over 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 V. 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, Dr. 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, Mass., 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.