Babies born just a few weeks early have a slightly higher risk of health problems in infancy, research suggests.
Doctors said their work challenged widely held views that babies born after 37 weeks had similar long-term outcomes to those born at full term.
The study in the British Medical Journal looked at 14,000 children, born 10 years ago, up to the age of five.
It looked at health outcomes including admissions to hospital and having illnesses such as asthma.
Previous work has focused on babies born very prematurely, before 32 weeks.
But this study suggests that the higher number of babies born later may also need extra attention.
It found that babies born before 39 weeks have a slightly higher risk of health problems up to the age of five. The earlier the baby arrived, the bigger the risk.
For example, 15% of babies born full term experienced asthma or wheezing as young children but the figure increased to 17% for those born just a few weeks early.
They were also slightly more likely to go into hospital.
About a fifth of babies - some 100,000 a year - are born early at 37-38 weeks.
The authors of the study were at pains to stress that parents should not worry about what was a modest chance of extra illness.
But the work should instead be used to question the level of healthcare support and monitoring given to these families, they added.
'Gradient of risk'
The research was carried out by the universities of Leicester, Liverpool, Oxford and Warwick and the National Perinatal Epidemiology Unit.
Dr Elaine Boyle, from the University of Leicester, said: "We've found that it's no longer appropriate, as we have done previously, to think of babies as either being born at term or premature.
"What we've found is that there is a gradient of increasing health risk with increasing prematurity but this risk stretches right up until the time at which a baby should be born."
Andy Cole, chief executive of the special care baby charity Bliss, welcomed the research.
He said: "This study highlights the need for the very best care to be given to all babies born preterm no matter at what gestation, and not just those admitted to intensive care.
"Babies born early are at a higher risk of conditions such as asthma in childhood and should be given regular health check-ups to ensure they remain healthy.
"While the study indicates a slight increase in the risk of asthma and wheezing in children born a few weeks early, we would not suggest this is a cause for concern."