(CBS) — Every year in the U.S. around a half a million babies are born premature. And while most grow up fine researchers say some can face serious complications into childhood, even death.

For babies born premature the first few weeks are critical.

"The pulmonary system hasn't fully developed, the central nervous system hasn't fully developed and these are very, very fragile,” explained Dr. Steven Schwarz of Long Island College Hospital of Brooklyn.

But a new study in the Journal of the American Medical Association finds a preemie’s risk can continue for many years.

"We were surprised that it persisted so long into childhood,” said Dr. Geeta Swamy of Duke University Medical Center.

Dr. Swamy followed 60,000 preemies born in Norway.

She discovered a significant increased risk of death throughout childhood, up to 13 years old in some cases.

"The highest risk was in those who were born extremely pre-term, and by extremely pre-term we define that as less than 28 weeks,” said Dr. Swamy.

Most pre-term babies will survive but some can face a lifetime of complications including asthma, allergies and learning disabilities.

Dr. Schwarz says there is a take-home message for physicians and parents.

“We have to watch closely and make sure problems don't develop and intervene early on, if we suspect something,” he said.

One discovery in this study may lead to a better understanding of what causes pre-term birth. Researchers found women who were born premature were more likely to have a preemie of their own.

"It is very possible or very plausible that there is some genetic or biologic phenomenon going on,” Dr. Swamy said.

The answer could be key to preventing prematurity.