Babies who are born premature have a higher risk of dying as young adults than those born full term, a surprising new study has found.

Premature babies often struggle with health problems in their first few months. But it has long been assumed that if they could survive, they were out of the woods and their risk of death into adulthood was no higher than those born at a healthy age.

But this new study finds the risk of early death returns decades later -- even among those who were born just a few weeks early.

The findings come from a study published in the Journal of the American Medical Association that looked at more than 600,000 babies born in Sweden between 1973-1979.

The researchers tracked deaths among the group and found that children born before 37 weeks of pregnancy were more likely to die before age five than others. That finding wasn't surprising given that previous research has shown preemies often struggle in their first years.

The researchers noted that the early death risk dropped off in late childhood and adolescence. But the risk re-emerged in early adulthood when the preemies grew to between 18 and 36 years.

For young adults born full-term -- between weeks 37 and 42 -- the death rate was 0.46 per 1,000 per year. For those born between 22 to 27 weeks' gestation, the death rate was 0.94 per 1,000 people per year.

The researchers noted that the more premature the babies were when they were born, the higher their risk of early death in young adulthood. But even among those born less premature -- between 34 and 36 weeks gestation -- their risk of death was still higher compared to those born full-term.

The study didn't closely examine the causes of death among the group, so it's unclear why preemies might have a higher death risk as young adults.

The researchers say that, to their knowledge, this is the first study to look at how gestational age at birth affected mortality in adulthood.

They say the findings are important given that in the past three decades, the prevalence of preterm birth in the United States has increased to more than 12 per cent, fuelled in large part by an increase in multiple births.

The authors say what needs further study is why being born premature might increase the risk of early death.

"The underlying mechanisms are still largely unknown but may involve a complex interplay of fetal and postnatal nutritional abnormalities; other intrauterine exposures, including glucocorticoid (a steroid hormone) and sex hormone alterations; and common genetic factors," they write.