There are now more Apple iPhones sold per second than there are babies born in the world.

In the December quarter, Apple sold 37 million of the world's most popular smartphone, at a rate of 4.6 per second.

This compares to the current global birth rate of about 4.2 births every second.

While the United Nations predicts the birth rate will soon climb to five births a second - as the global population surpasses 7 billion - the rate of smartphones sales is likely to grow even faster.

Foad Fadaghi, a research director at technology analysis firm Telsyte, says phone sales are not likely to slow any time soon.

''We are going to see the developing world adopting smartphones at a very rapid rate over the next three to four years ... particularly in countries where there is a lack of fixed-line infrastructure.''

He says a reduction in prices, where you can now buy a very basic smartphone for under $100, is helping to drive the uptake.

While mobile phone use is much closer to saturation point in the Western world, Fadaghi says sales will remain strong, driven by consumers updating their smartphones at an ever-increasing rate.

''Consumers used to purchase a new mobile phone every three to four years,'' he says. ''Today, on average, they are purchasing a new phone every two years or less.''

Fadaghi says the faster rate of replacement is being driven as much by people's desire to have the latest and greatest device as by the limited lifespan of smartphones.

According to research by warranty provider SquareTrade, more than 25 per cent of iPhone owners experienced a failure in the first two years of use.

''Two to three years is the usual period these things will last in terms of the technology and the lifetime of the hardware itself,'' Fadaghi says.

''But consumers will often replace their phone a lot sooner. With iPhones and other models being updated every 12 to 18 months now, many consumers want the latest technology as soon as it's available.''