A nurse looks after a premature baby inside an incubator at an Egyptian public hospital in the province of Sharkia to the northeast of Cairo on June 10, 2008. Credit: Reuters/Nasser Nuri

They found that extremely pre-term babies who lived to age 11 often had abnormal lung function and were twice as likely as children born at a full 39 or 40 weeks to be diagnosed with asthma.

Many of these children may not be receiving appropriate treatment, said Janet Stocks of University College London, who led the study.

Stocks and colleagues used data from a large study that tracked all babies born in Britain and Ireland at or before 25 weeks gestation between March and December 1995.

The children were examined and their lung function and respiratory health checked when they were 2, 6 and 11. They were compared to children of the same age, sex and ethnic origin.

More than half, or 56 percent, had abnormal spirometry results -- a test of blowing into an instrument to assess lung strength, they reported in the American Journal of Respiratory and Critical Care Medicine.

One in four had asthma. But 65 percent had not had any respiratory symptoms for the past 12 months.

As the lungs grow and airway caliber increase, such children will be far less prone to wheezing episodes and will appear to have 'grown out' of their symptoms, Stocks said in a statement.

However, there is concern that such symptoms may reappear in later life in the form of early onset chronic obstructive lung disease, she added.

Even if asymptomatic, those who enter adulthood with impaired lung function will be at increased risk of subsequent chronic lung disease. There is therefore a continuing need to monitor these children for respiratory problems.

As medical technology allows more and more pre-term children to survive, this could become more of a problem, and something for healthcare planners to be aware of.