While investment over the last decade in health care for women and children has paid off in rapid declines in maternal death rates and deaths of children under five, improvement in the survival of babies in their first four weeks of life has been slower.
"Newborn survival is being left behind despite well-documented, cost-effective solutions to prevent these deaths," said Flavia Bustreo, a WHO expert in family, women's and children's health who worked on the study.
According to the findings, newborn deaths decreased overall from 4.6 million in 1990 to 3.3 million in 2009, but began falling slightly faster from 2000 to 2009.
Deaths of babies in their first four weeks of life now account for 41 percent of all child deaths before the age of five, a share which has grown from 37 percent in 1990 and is likely to increase further, the researchers said.
Yet the three leading causes of newborn death -- preterm delivery, asphyxia and severe infections -- are all relatively easily preventable with proper care.
Joy Lawn, of the charity Save The Children, who also worked on the study, said a critical global shortage of trained healthcare workers was a major contributing factor.
"We know that solutions as simple as keeping newborns warm, clean and properly breastfed can keep them alive, but many countries are in desperate need of more and better trained frontline health workers to teach these basic lifesaving practices," she said. "Training more midwives and more community health workers will allow many more lives to be saved."
The study, which covers 20 years and all 193 WHO member states, was carried out by researchers from the WHO, Save The Children and the London School of Hygiene and Tropical Medicine. It was published in the PloS (Public Library of Science journal) Medicine on Tuesday.
It found that almost 99 percent of newborn or neonatal deaths -- deaths in the first four weeks of life -- occur in the developing world. It also found that, in part because of their large populations, more than half of these deaths happen in five countries -- India, Nigeria, Pakistan, China and Democratic Republic of Congo.
Afghan babies face the greatest risks -- with one in 19 dying in the first month of life. India has more than 900,000 newborn deaths per year, nearly 28 percent of the global total.
Nigeria, the world's seventh most populous country, now ranks second in newborn deaths up from fifth in 1990, while China moved from second to fourth place.
With a reduction of only 1 percent per year, Africa has seen the slowest progress of any region in the world.
Among the 15 countries with more than 39 neonatal deaths per 1,000 live births, 12 were in Africa. At the current rate of progress it would take the African continent more than 150 years to reach the level of newborn survival rates seen in the United States or Britain, the researchers said.
"This study shows in stark terms that where babies are born dramatically influences their chances of survival -- and that especially in Africa, far too many mothers experience the heartbreak of losing their baby," said Lawn.