When the researchers accounted for several other factors - like family income and whether the mother was on asthma medication - extreme preterm birth was linked to a likelihood of adult asthma 2.4 times that seen in the full-term group.

In the United States about 7.3 percent of adults (16 million) and 9.4 percent of kids under 18 (7 million) have asthma, according to the Centers for Disease Control and Prevention. About $30 billion is spent annually to treat the condition, including inhalers and other drugs as well as hospitalizations.

The results of the current study do not prove that very early birth, itself, leads to asthma in some adults.

But the connection is plausible, according to Crump.

“Preterm delivery can result in immature lung development, altered immune function that is needed for normal lung function, and increased susceptibility to infection or environmental factors such as smoking,” he explained.

Parents of very-preterm children, and young adults who were born very early, should be aware of the risk, Crump said. That includes having any respiratory problems checked out promptly.

And, Crump said, it’s “very important” that young people avoid smoking, which would further boost any increased asthma risk.

It’s not clear, though, whether the asthma risk seen in this study group will necessarily hold true for infants born in recent years.

“It’s possible that their long-term risk may be different due to changes in neonatal care,” Crump said, “but it will take years to answer this question.”

SOURCE: Pediatrics, online March 21, 2011.