Babies born as little two or three weeks before their 'due date' have a higher risk of health problems, such as asthma, according to new research.
The study conducted by researchers at Leicester, Liverpool, Oxford and Warwick Universities and the National Perinatal Epidemiology Unit, challenges the long-held belief that a baby is 'full term' from 37 weeks.
The researchers looked at health outcomes in 14,000 children born 10 years ago and found that babies born before 39 weeks have a slightly higher risk of health problems up to the age of five.
The findings, published in the British Medical Journal (BMJ), also suggested the earlier the baby arrived, the bigger the risk.
It was found that 17% of babies born a few weeks early had experienced asthma or wheezing as young children compared to 15% of those born full term.
However, the study authors stressed that the findings should not cause concern among parents as the results showed only a slight increase in risk.
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."
Experts believe the findings should be used to question the level of healthcare support and monitoring given to the families of children born before 39 weeks.
Andy Cole, chief executive of the special care baby charity Bliss, told the BBC: "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."
Leanne Metcalf, assistant director of research at Asthma UK, said, as reported by the Press Association: "This is not the first piece of research to indicate that every week spent in the womb is important for a baby in order to reduce its risk of developing asthma in childhood.
"The advantage of this study, however, is its scale in terms of the number of children whose asthma development compared to their gestational age has been measured, and the fact that it has looked at babies who are born just a few weeks prematurely."
She said it was reasonable to believe the gradual development of the lungs might influence the risk of asthma.
"There are a number of things that pregnant women can do to reduce the risk of prematurity in their baby, including maintaining a healthy weight, staying active and avoiding stress, smoking and infections.
"Obesity, stress and smoking have also all been separately linked to an increased risk of asthma in children, smoking especially, so taking steps to avoid them will enable pregnant women to give their baby the best possible chance of a healthy childhood."
To increase your baby's chances of a healthy start in life, see our roundup of the best foods to eat during pregnancy...
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