Young adults who were born very premature may have an increased risk of asthma, a report published Monday suggests.

The study, of more than 600,000 Swedish adults born in the 1970s, found that those born very early - between the 23rd and 27th weeks of pregnancy - were more than twice as likely to have asthma as young adults born full-term.

There was, however, no increased risk seen among those born after the 27th week of pregnancy.

A full-term pregnancy lasts about 40 weeks; any birth before week 37 is considered preterm.

The findings, reported in the journal Pediatrics, are “somewhat unexpected,” said lead researcher Dr. Casey Crump, an assistant professor of medicine at Stanford University in California.

That’s because previous research had suggested that respiratory problems after preterm birth generally wane during childhood, Crump told Reuters Health in an email.

However, he said, those studies mainly included young people who had been born moderately premature. Historically, very few extremely premature infants survived. So, only now are there enough extreme preemies who have made it to adulthood that large studies can look at their long-term health picture.

The new study is the first one to examine a big enough population to show a connection between extreme prematurity and long-term asthma risk, according to Crump’s team.

The findings are based on data for all 622,616 singleton infants born in Sweden between 1973 and 1979.

Overall, four percent were born prematurely - including 165, or 0.03 percent, who were extremely preterm.

Of those 165, 9 percent had asthma drug prescriptions at some point between 2005 and 2007, when they were 25 to 35 years old. That compared with roughly 4 percent of both the full-term and moderately premature birth groups.