By Alex Kirby
BBC News Online environment correspondent
Efforts to slow down the growth of the world's population by linking poverty relief to women's rights and access to birth control are working, the UN says.
The UN says further progress is being hindered
But it says that a lack of money, gaps in provision and gender bias are making further progress difficult to achieve.
The UN Population Fund (UNFPA) uses its 2004 annual report to sound a warning about the mounting challenges it sees.
The report is a review of progress made since the 1994 international conference on population and development in Egypt.
The State Of World Population 2004 is entitled The Cairo Consensus At Ten: Reproductive Health And The Global Effort To End Poverty.
It says the Cairo plan, adopted by 179 countries, sought to balance the world's people with its resources, improve women's status, and ensure universal access to reproductive health care, including family planning.
The plan "gave priority to investing in people and broadening their opportunities, rather than to reducing population growth."
Among the changes the report notes is an increase in the use of modern contraception, from 55% of couples in 1994 to 61% today. But it says about 201 million women still have an unmet need for effective contraception.
It says: "Meeting their needs would cost about $3.9bn a year, and prevent 23 million unplanned births, 22m induced abortions, 142,000 pregnancy-related deaths and 1.4 million infant deaths."
Implementing the Cairo agenda costs money, it says, but resources are short. Donors agreed in 1994 to provide $6.1bn a year for population and reproductive health programmes by 2005, a third of the total needed.
In 2002, the last year for which figures are available, contributions were about $3.1bn, barely half the total promised.
In 2003 developing countries themselves were spending about $11.7bn on the Cairo agenda. But much of that was spent by a few large countries, leaving the poorest ones heavily dependent on outside help.
The reality means more than 350 million couples still lack access to a full range of family planning services, and 529,000 women die annually from complications of pregnancy and childbirth, most of them preventable.
Five million new HIV infections occurred in 2003: nearly half of all infected adults are women, and nearly three-fifths of those in sub-Saharan Africa.
In some areas in the region 25% of the workforce is HIV-positive. The report says studies show that if 15% of a country's population is HIV-positive, its GDP will decline by 1% annually.
The report spells out some of the pressures of population growth:
The report says the Earth's population is likely to increase from 6.4 billion people today to 8.9 billion by 2050, with the 50 poorest countries tripling in total numbers to 1.7bn people.
- about 2.8 billion people, two in five of those alive, survive on the equivalent of less than $2 a day
- about half the world's original forests have now been cleared, and three-quarters of its fish stocks are being exploited at or beyond sustainable limits
- half a billion people live in countries where water is scarce: by 2025, the number is expected to be 2.4-3.4 billion people
- a majority of the world's people will be living in cities by 2007, and by 2030 all regions will have urban majorities.
It says poverty dramatically increases a woman's chances of dying: "The lifetime risk of a woman dying in pregnancy or childbirth in West Africa is 1 in 12. In developed regions, the comparable risk is 1 in 4,000."