The study appears online today in JAMA Network Open.

About 10% of all births worldwide are classified as preterm, which occurs when a baby is born before 37 weeks gestation. Globally, preterm birth is the leading cause of infant mortality and the second leading cause of death for children under the age of 5.

“Understanding the long-term effects of preterm birth can help us develop preventative strategies and identify interventions to improve the health of individuals who are born preterm,” said Asma M. Ahmed, M.D., Ph.D., assistant professor of epidemiology and prevention at Wake Forest University School of Medicine, former post-doctoral fellow at SickKids, and the study’s lead author.

Ahmed said while the majority of people who are born preterm survive into adulthood, there’s growing evidence that highlights an increased risk of death throughout life.

“We wanted to know whether people born prematurely have a higher risk of short-term and long-term mortality compared to individuals who were born full term,” Ahmed said.

In the first known population-based study of preterm birth in North America, Ahmed and team analyzed a cohort of nearly 5 million live births in Canada of which 6.9% were preterm. The participants were born between 1983 and 1996, and were tracked through 2019, providing a follow-up period ranging from 23 to 36 years.

The preterm births were categorized into gestational age subcategories: 24-27 weeks, 28-31 weeks, 32-33 weeks and 34-36 weeks and compared with the full-term births, 37-41 weeks.

The researchers found that preterm birth is associated with increased risk of death across all age groups up to the age of 36 years, with the highest risks occurring from birth to infancy, 0-11 months, and early childhood, 1-5 years.

“We also found that the risk of mortality is higher with a lower gestational age at birth, with those born before 28 weeks facing the highest risks,” Ahmed said.

The study also identified increased risks of mortality associated with several causes such as respiratory, circulatory and digestive system disorders; nervous system, endocrine and infectious diseases; cancers; and congenital malformations.

“These findings suggest that preterm birth should be recognized as a significant risk factor for mortality,” Ahmed said. “The risk remains until the third and fourth decades of life. While clinical care during the neonatal period is critical, long-term follow-up care and monitoring for people born preterm is crucial to help mitigate the risks.”

Ahmed said additional research is needed in other populations, especially in low-income and middle-income countries where preterm birth rates are highest. She also highlighted the importance of research that identifies underlying factors contributing to the increased mortality risk after preterm birth.

The study was supported by the Canadian Institutes of Health Research grant No. 438541.