Premature birth is the leading cause of infant mortality in developed countries, but far less is known about what long-term impact -- if any -- being born before 37 weeks can have on a person's health.
Now, a new study suggests that preterm babies have a higher risk of mortality in their late teens, 20s and into their 30s than their full-term counterparts. And the earlier a baby is born, the greater his or her risk of death in early adulthood.
Researchers writing in the Journal of the American Medical Association looked at records from more than 600,000 individuals born in Sweden in the 1970s -- 5 percent of whom were born preterm -- and followed them up to 2008.
They found that premature babies who made it through their first year of life had a higher risk of dying in early childhood, up to age 5. As they aged, that risk waned: from age 6 to 17, the risk of mortality was on par with that of full-term babies. However, by young adulthood, age 18 to 36, the increased risk of death reappeared.
"This was due to several different causes, including congenital anomalies and respiratory, endocrine and heart disease," Dr. Casey Crump, a clinical professor of medicine at Stanford University and one of the study's lead authors, wrote in an email to HuffPost. "Some of these causes may have long latency ... or may have cumulative effects that don't manifest until early adulthood."
Outside expert Dr. Arthur D'Harlingue, a neonatologist at Children's Hospital and Research Center Oakland, said that brain development can play a crucial role in the short- and long-term health of premature babies -- a role that is not yet fully understood. For many years, experts assumed that 10 weeks after birth, a baby born 10 weeks early would have a brain similar to that of a full-term newborn.
"But the brain develops differently when it's exposed to different nutrition, to a different environment and to different stresses after birth," he said. "Brain MRIs of premature infants are finding all kinds of differences, which are not always necessarily bad, but they are there."
According to the authors of the new study, premature births are up in the U.S., increasing to 12 percent of all births over the past three decades.
D'Harlingue cited multiple medical factors as the cause, including increased obesity and diabetes among mothers, as well as lifestyle issues like drug abuse and lack of prenatal care. Elective cesarean sections play a role, he explained.
The March of Dimes, an organization that works to combat premature birth, has questioned the role medically unjustified C-sections may play in the risk for prematurity. "Women should wait until at least 39 weeks to schedule an induced labor or C-section, unless there are medical problems that make it necessary to deliver earlier," its website states.
But the new study's authors say that premature birth should not be a source of fear for would-be parents.
"Most young adults who were born preterm have a high level of function and quality of life," Crump told HuffPost. "Although we found that preterm birth was linked with increased mortality in young adulthood, the absolute risk of dying was still low -- less than 1 per 1,000 people per year."
As for parents of premature babies, and those individuals born prematurely, understanding the possible risks, he said, is beneficial:
"It's even more important for survivors of preterm birth to avoid other risk factors for disease such as smoking and obesity to offset the increased risks that we found."