Being born preterm needs to be considered a chronic condition, and early arrivals would benefit from monitoring throughout life, a US doctor says in light of findings on mortality and premature birth.

A study of more than 6 million people from four Nordic countries has found all degrees of prematurity are associated with an increased risk of death between the ages of 15 and 50 years.

Being born preterm birth is associated with 2-fold increased risk of death for each of cardiovascular disease, diabetes and chronic lung disease, the study in JAMA Network Open shows.

Those born early preterm (before 34 weeks) had 1.4 fold mortality compared with those born full term (39-41 weeks).