The world's 7 billionth person will be born into a population more aware than ever of the challenges of sustaining life on a crowded planet but no closer to a consensus about what to do about it.

To some demographers the milestone foreshadows turbulent times ahead: nations grappling with rapid urbanisation, environmental degradation and skyrocketing demand for healthcare, education, resources and jobs.

To others, a shrinking population, not overpopulation, could be the longer-term challenge as fertility rates drop and a shrinking workforce is pushed to support social safety for an ageing populace.

"There are parts of the world where the population is shrinking and in those parts of the world, they are worried about productivity, about being able to maintain a critical mass of people," Babatunde Osotimehin, executive director of the UN Population Fund, told Reuters.

"Then there are parts of the world where the population is growing rapidly. Many of these countries face challenges in terms of migration, poverty, food security, water management and climate change and we need to call attention to it."

The United Nations says the world's seven billionth baby will be born on October 31.