There are nearly 13 million premature births worldwide each year, and of these more than a million die within a month of birth, according to a report by a US charity.
The problem mainly affects developing nations, with Africa and Asia accounting for more than 85 percent of all premature births, though US premature births have increased 36 percent over the past 25 years, according to research by the US group the March of Dimes.
For purposes of the study, premature is considered as less than 37 full weeks of gestation.
While malnutrition and poor health care can explain many of the premature births in developing countries, factors in the United States include more women becoming pregnant beyond the age of 35, and more using assisted reproduction techniques, say the report authors.
"Premature births are an enormous global problem that is exacting a huge toll emotionally, physically, and financially on families, medical systems and economies," said March of Dimes president Jennifer Howse on Sunday.
"In the United States alone, the annual cost of caring for preterm babies and their associated health problems tops 26 billion dollars annually," Howse said in a statement.
"If world leaders are serious about reaching the United Nation's Millennium Development Goals to reduce child mortality and improve maternal health, then strategies and funding for reducing death and disability related to preterm birth must receive priority," Howse said.
Mario Merialdi, one of the authors of the WHO bulletin with the latest figures and an editor of the March of Dimes study, described the report as "a first attempt to estimate the worldwide scale of the problem.
"As a first step, it is necessary to improve data on the extent of the problem," Merialdi said, adding that the WHO is improving its database on preterm birth.
Another problem is the lack of a widely accepted classification of preterm birth or glossary of terms, said Joy Lawn with the group Saving Newborn Lives/Save the Children USA.
"We need to at least adopt common definitions and agree on what is a preterm baby," said Lawn, adding that more national and regional data on the long-term health problems caused by preterm birth is needed.
The data is to be presented this week at the 4th International Conference on Birth Defects and Disabilities in the Developing World in New Delhi, the March of Dimes said.