Of all the issues that will be aired from the pulpit of the UN's development summit this week, one is likely to stand out by its absence: What should be done about the world's population explosion?
To many campaigners, demographic growth is the gorilla in the UN's living room, a blindingly obvious problem interlinked with poverty and environment that gets carefully ignored whenever leaders meet.
"When the Millennium Development Goals were adopted, there was not a single target on population or family planning access," said Alex Ezeh, executive director of the Africa Population Studies and Health Research Centre in Nairobi.
"It was a huge mistake," he said. "The world is only now just waking up."
Campaigners on population issues acknowledge that poverty and environmental damage can have complex causes. A surge in population in some well-documented cases has helped catapult a country to prosperity.
But, they also say, relentless population pressure is common to many of the problems besetting the Millennium Development Goals (MDGs), up for review in New York. The three-day summit opens on Monday.
In poor countries, unbraked demographic growth adds to strain on infrastructure, health and educational resources, amplifies the risk of environmental damage and boosts exposure to the wrath of climate change.
Ezeh pointed in particular to the dilemma in Africa.
Even if countries reduce the proportion of people living in poverty, the number living in poverty grows in absolute terms simply because of massive population growth.
"If you have a population growing at three percent (per year), that means it is doubling every 23, 24 years or so," he said.
"It means, for instance, that you will have maybe twice the number of children needing education. But it is almost impossible for countries to double the number of schools and double the number of teachers during this time."
One example is Kenya, where the population in 2009 stood at 38.6 million, an increase of around 10 million since 1999. Less than a third of Kenyans have piped water and three-quarters have no mains sanitation.
Since the MDGs were drawn up in 2000, the world's population has expanded from 6.0 to 6.8 billion, 95 percent of whom were born in poorer countries. By 2050, the total is likely to be more than nine billion, according to UN estimates.
Providing these extra souls with housing, water, electricity, sewerage, hospitals and schooling is going to be a mighty challenge, as a report issued in March by UN Human Settlements Programme revealed.
It found that 227 million people had escaped slums in the past decade -- but the overall people living in slums had increased, from 776.7 million to 827.6 million. Half of the rise was due to population increase in existing slums, and a quarter to rural exodus.
Even tiny investments such as access to contraception, improvements in sex education and promotion of reproductive rights for women can bring big benefits, say advocates.
Smaller families -- achieved gradually, and through voluntary measures -- mean lower overheads, less environmental damage, reduced exposure to the risks from climate change, and children that have a better chance of a healthier life.
"Each dollar spent on family planning can save up to 31 dollars in health care, water, education, housing and other costs," according to an estimate in the latest issue of the US journal Science, written by eight experts in public health, conservation and development.
Politically, though, commitment on population issues has been weak, absent or deliberately stifled at the top level, say critics.
The UN's World Summit on Sustainable Development in 2002 declared it would haul billions of people out of poverty and protect the environment.
Yet it made no mention of how to achieve these goals when the planet's population was expected to rise by 50 percent within half century.
Similarly, the original set of MDGs made not a single reference to population.
Responding to an outcry, leaders tacked on a target, MDG 5b, for achieving "universal access to reproductive health" by 2015. It joined the other MDGs in 2007.
"Because we started seven years late, the result is hugely disappointing. There are 215 million women who have an unmet need for family planning," said Gill Greer, director-general of the International Planned Parenthood Federation (IPPF) in London.
Global spending on family planning halved because of its absence from the MDGs, and even now "there is still a reluctance" to raise the issue, she said.
Seeking to explain why population has failed to feature on summit radar screens, suspicions fall partly on the Roman Catholic church and evangelists in former US president George W. Bush's administration, said Greer.
But another explanation could lie in bad memories of coercive measures -- forced sterilisations in India in the 1970s, and controversies arising from China's one-child policy, which among other things has skewed gender numbers hugely in favour of boys.
Andrew Dorward, professor of development economics at the School of Oriental and African Studies in London, said it was too late for population policies to have much of an impact on the current MDGs, whose target date is 2015.
"Where it will have an effect is 10, 15, 20 years from now," he said.
"We can start acting now, in a positive, supportive way. It's no good saying that getting to nine billion (people) in 2050 is going to be a problem. You've got to start working on it now."