TUESDAY, Sept. 20 (HealthDay News) -- Young adults who were born prematurely may not give the circumstances of their birth much thought, but a new study finds that health problems may re-emerge decades later and raise their risk of death for some time.

From 18 to 36 years of age, people who were born preterm face up to twice the risk of death compared to young adults born at a normal gestational age, the researchers found. They also noted that preemies have an increased risk of death from birth until age 5 years, but that association was less surprising.

"We found that people who were born preterm had a higher risk of dying than people born full-term. Even [those born] a couple of weeks early had an increased mortality in young adulthood," said the study's lead author, Dr. Casey Crump, a clinical assistant professor of medicine at Stanford University in California.

"In young adulthood, each additional week of pregnancy resulted in a 4 percent reduced risk of dying as a young adult," he added.

However, any individual's risk of dying was still low. Even those who were born between 22 and 27 weeks' gestation had less than a 1 percent risk of dying as a young adult, according to the study, published in the Sept. 21 issue of the Journal of the American Medical Association.

Dr. Kenneth Bromberg, chairman of pediatrics at the Brooklyn Hospital Center in New York City, said that on an individual level, those who might be more at risk may already be aware of that risk. "Some of these kids with congenital anomalies, like heart defects, are already aware of their problems. What we can't tell from this kind of large population data is if there are other ill-defined problems," he said.

Besides congenital abnormalities, the association was seen with deaths from respiratory, cardiovascular and endocrine problems, but not cancer, neurological disorders or accidents, the authors said.

Any birth that occurs before 37 completed weeks of pregnancy is considered preterm, according to the study. Preterm births are the leading cause of infant mortality, but more and more preterm babies are surviving, the study authors noted. Yet, little information is available on what happens as these babies reach adulthood, they added.

The current finding comes from a Swedish study of al;most 675,000 people born between 1973 and 1979. Of these, nearly 28,000 were born preterm. The study followed the babies through 2008, when they were between 29 and 36 years old.

A total of 7,095 deaths occurred in the entire group. The researchers found a strong inverse association between each week of pregnancy and the risk of death for two age groups: children between 1 and 5 years old, and young adults (aged 18 to 36 years). No association was found between risk of death and the number of weeks of pregnancy for children between the ages of 6 and 17 years.

For the younger group, each additional week of pregnancy was associated with an 8 percent decreased risk of dying. For the young adults, that number was 4 percent for each additional week of pregnancy.

The incidence of death for young adults was 0.94 per 1,000 person-years for babies born between 22 and 27 weeks' gestation. For babies born between 28 and 33 weeks' gestation, the rate was 0.86 per 1,000 person-years. At 34 to 36 weeks' gestation, the rate was 0.65 and at normal gestation (37 to 42 weeks), the rate was 0.46 per 1,000 person-years, according to the study.

The researchers don't know exactly why there's an increased risk of death in adulthood, after a relatively long period where death risk isn't increased. "There is an increased risk of health problems in premature infants. Maybe some of these conditions have a long latency or period of development," suggested Crump.

Bromberg also noted that some of the life-saving interventions done to premature babies might cause problems as the children get older.

The management of premature infants has changed over the years, but Crump said it isn't clear how, or even if, newer therapies might change these findings.

"There needs to be better awareness of the long-term health effects of preterm birth. The early health effects are well-known, but it's important to be aware of long-term risks of preterm birth," said Crump.

More information

Learn more about why babies are born prematurely from the March of Dimes.