Premature babies have higher risk that normal term infants of dying before the age of five
Being born prematurely increases the risk of dying early as a young child or adult, a study has shown.
Scientists looked at data on more than 600,000 people born in Sweden between 1973 and 1979.
A "strong inverse association" was found between gestational age at birth and death between the ages of one and five.
In other words, children were more likely to die in their first five years the more prematurely they were born.
The pattern faded away in late childhood and adolescence, but returned in young adulthood between the ages of 18 and 36.
During this age period, individuals were again more likely to die if they were born pre-term.
In young adulthood, death rates per 1,000 "person years" ranged from 0.95 for those born most prematurely to 0.46 for those born after a full-term pregnancy.
A person year is the sum of all the years each member of a study population has been under observation.
The findings were published today in the Journal of the American Medical Association.
The researchers, led by Dr Casey Crump, from Stanford University in California, wrote: "The underlying mechanisms are still largely unknown but may involve a complex interplay of foetal and post-natal (post birth) nutritional abnormalities, other intra-uterine exposures including glucocorticoid (a steroid hormone) and sex hormone alterations, and common genetic factors."
They added: "Clinicians will increasingly encounter the sequelae (consequences) of pre-term birth throughout the life course and will need to be aware of the long-term effects on the survivors, their families and society."