By Helen Briggs
Health reporter, BBC News
Progress in reducing maternal deaths has been slow
Maternal deaths have fallen worldwide, from about half a million a year in 1980 to less than 350,000 in 2008, according to new data.
Countries such as China are making significant progress but there have been surprising increases in others, including the US, say researchers.
UK deaths are very low, but have not fallen in the past 20 years, the study, published in the Lancet, found.
Making childbirth safe for all women has long been an international goal.
But progress in some countries has been slow.
In the latest study, a team led by the University of Washington in Seattle, looked at data from thousands of observations of maternal deaths for 181 countries between 1980 and 2008.
They estimated there were 342,900 maternal deaths worldwide in 2008, down from 526,300 in 1980.
More than half of all maternal deaths were in only six countries in 2008 - India, Pakistan, Nigeria, Afghanistan, Ethiopia, and the Democratic Republic of the Congo.
But some countries - including China, Egypt, Ecuador and Bolivia - had made significant progress towards achieving international goals on maternal mortality.
Lead author Dr Christopher Murray said: "There are still too many mothers dying worldwide, but now we have a greater reason for optimism than has generally been perceived."
He said finding out why a country such as Egypt has had "such enormous success in driving down the number of women dying from pregnancy-related causes could enable us to export that success to countries that have been lagging behind".
The picture in high-income countries is less clear. One of the most surprising findings was an increase in the number of expectant mothers dying in the US, from 12 in every 100,000 live births in 1990, to 17 in 2008.
The authors say the trend can be explained in part by changes in the way maternal deaths are recorded in the US.
In the UK, maternal mortality rates fell between 1980 and 1990, and then levelled off - which reflects the trend in most western European countries.
The rate per 100,000 live births in the UK is eight, with Germany and Spain at seven, and France at 10.
Commenting on the statistics, Cathy Warwick, General Secretary of the Royal College of Midwives, said it was clear that around the world needless deaths can be avoided.
But she expressed concern that the UK rate - although very low - is not falling.
She added: "It is possible that this is due to increasing levels of ill health amongst pregnant women and possibly to greater numbers of older women giving birth."
Lancet editor Dr Richard Horton said there was a dramatic difference between the latest estimates and those last reported by the UN.
He added: "Two decades of concerted campaigning by those dedicated to maternal health is working.
"Even greater investment in that work is likely to deliver even greater benefits. Women have long delivered for society, and, slowly, society is at last delivering for women. This is a moment to celebrate - and accelerate."