Population boom set to stabilise at 9bn by 2300.
UN study predicts falls in fertility but greater life expectancy.

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But the report, issued by the UN's population division, gives warning that even slight variations - "as little as one-quarter of a child" - below or above this two-child norm could produce dramatic swings, resulting in world totals ranging from as little as 2.3 billion up to 36.4 billion. If fertility levels remained unchanged at today's levels, the current world population of 6.4 billion would rise to 44 billion in 2100, 244 billion in 2150 and 1.34 trillion in 2300, the UN says.