One in 10 of the some 130 million births around the world each year is premature, the vast majority in poorer countries where chances of survival are low, the World Health Organization (WHO) said on Monday.
An article in the U.N. agency's January bulletin also reported a dramatic rise in pre-term births in a range of richer countries over the past 20 years, especially in North America and parts of Europe.
Based on studies from the mid-1990s to 2007, it said 85 percent of births before the normal 37-week human gestation period were in Asia, with some 70 million, and in Africa, with more than 40 million annually.
But the highest rates of pre-term deliveries against the overall total of births were in Africa, with an average of nearly 12 percent, and North America, with 10.6 percent, according to the article by WHO specialists and researchers.
In Europe, the figure was only 6.2 percent and in Latin America and the Caribbean just 9.1 percent.
Many premature babies in Asia and Africa have no access to effective care, said Dr Lale Say, a lead author of the article. One born at 32 weeks, weighing less than 2,000 grams, has little chance of survival, the WHO specialist wrote.
By contrast, an infant born at 32 weeks in a developed country is as likely to survive as one born at full term.
Evidence from the United States shows that about 50 percent of babies born after gestation periods as short as 22-25 weeks may survive, half of them without serious health problems by the time they reach 18 to 22 months of age, the article said.
The large numbers of premature births in Asia -- mainly in and around the Indian sub-continent where the average is 11.4 percent -- and Africa, where in the southern region it reaches 17.5 per cent, were largely due to absence of drugs to treat infections suffered by mothers during pregnancy.
But in the richer countries increasing numbers reported not just from the United States but also from Britain and the Scandinavian countries suggested modern medical technologies and lifestyle changes played a role.
Increasingly women over 34 in countries with advanced health systems were choosing to have babies. Other factors were greater use of assisted reproduction techniques and greater numbers of mothers opting for Caesarean sections.