'Tragedy' of maternal death
Women in poor nations are 300 times more likely to die in childbirth or from pregnancy complications than those in the developed world, Unicef warns.
The lifetime risk in a developing country was one in 8,000, compared with one in 24,000 in richer countries.
About 99% of the 500,000 maternal deaths in 2005 occurred outside industrialised nations, more than half of them in Africa, Unicef said.
Its head of health said there were an "unconscionable number of deaths".
In Niger, the country with the world's highest maternal mortality, a woman has a one in seven chance of dying, during pregnancy or childbirth.
Ireland - where the risk of death is one in 47,600 - is the safest place to have a baby.
The United Nations Children's Fund says in its ' The State of the World's Children 2009' report that progress has been made in reducing deaths in under-fives.
The UN has called for a 75% reduction in the maternal mortality rate by 2015 as part of its Millennium Development Goals programme.
But Unicef says that nations, especially in the developing world, are falling far short of this mark.
It adds that girls who give birth before the age of 15 are five times more likely to die in childbirth than women in their 20s, the agency said.
In its report, Unicef said: "The divide between industrialised countries and developing regions - particularly the least developed countries - is perhaps greater on maternal mortality than on almost any other issue."
Dr Peter Salama, the Unicef chief of health, added: "It's really an unconscionable number of deaths. It's a human tragedy on a massive scale."
The number of maternal deaths has remained largely unchanged over the past two decades and has made it more difficult to reduce child mortality.
A newborn baby has less chance of surviving if its mother dies during or shortly after childbirth. Those born in developing countries are almost 14 times more likely to die during the first month of life.
Liberia had the highest rate of neonatal mortality at 66 deaths per 1,000 live births.
Ann Veneman, Unicef's executive director, said: "Progress has been made in reducing child mortality, but much more must be done especially in addressing maternal and newborn health."
She added: "Saving the lives of mothers and their newborns requires more than just medical intervention.
"Educating girls is pivotal to improving maternal and neonatal health and also benefits families and societies."
Unicef said up to 80% of the deaths had access to basic health and obstetric care.
About four in 10 of all births worldwide are not attended by a doctor or other health professional.
Improved access to HIV drugs would also improve the chances of survival for new mothers and their babies, it added.