Too many babies born today are arriving premature and too small – 12.5 percent of all births in 2004 – and nobody knows why, a new report has found.

The cost to society to care for preemies is $26 billion each year, according to the national Institute of Medicine, which issued the 600-page report Thursday calling for more research to reduce pre-term births.

“Despite great strides in improving the survival of infants born pre-term, little is known about how pre-term births can be prevented,” said Richard E. Behrman, chairman of the committee that wrote the report, and chairman of the Federation of Pediatric Organizations.

The report found the rate of premature birth has increased by more than 30 percent in two decades, and now accounts for more than 500,000 babies a year who are born at least three weeks early.

Behrman said part of the increase in pre-term babies is because women who take fertility drugs or undergo in-vitro fertilization often become pregnant with more than one baby, and most multiple births are early – 61.7 percent of twins and 97.2 percent of triplets and other multiples.

The Institute of Medicine said racial and economic factors play a role in a percentage of premature births, as does the age of the mother, but that in many cases there are no apparent risk factors. Women who are healthy, eat right and follow doctors’ orders can still go into labor and deliver early.

Dr. Victor Gonzalez-Quintero, director of Maternal Fetal Medicine at the University of Miami Miller School of Medicine, said more research is needed because most women who deliver early have no known risk factors.

“What is very concerning is that the United States has one of the highest rates (of pre-term births) among developed nations. It’s a very complex condition, but I think this kind of initiative will get the public’s attention in a manner that will prompt greater action.

Gonzales-Quintero said some research has been done over the past decade, but the issue is very complex.

“The only thing we’ve been able to find effective is the use of progesterone, if she’s had a previous pre-term baby,” Gonzalez-Quintero said. Giving the hormone has been shown to prevent early delivery in subsequent births, he said.