Hello and welcome to more or less on the BBC World Service, investigating the numbers in the news and in life.

I'm Jan Wong. This week...

Just how uncommon is it for a woman to give birth without knowing she was pregnant?

But first, and still in a roundabout way on the subject of babies, the United Nations Population Fund,

UNFPA, recently published a report describing the world's aging population.

Today we have one in 9 persons, 860 and over, but by 2050 it'll be 2 billion persons, 1 in 5 will be 60 and over.

And by that time, there will be more older persons than those under age 15 years.

And this is no longer just a developed country phenomenon. It's happening faster in fact in developing countries,

but most of them are not really prepared for the growing numbers of older persons.

That's Dr. Anne Paulicco from the UNFPA. She says we can be pretty confident about the numbers.

We can be very certain about the numbers for 2050, because persons who will be aged 60 in 2050 are already born, so this is not speculation.

Fair enough. But in the longer term, the figures aren't going to be dominated by the likes of you and me.

It's the birth of new people that will have the largest effect on future statistics. And here's where it gets interesting.

For a long time, statisticians have observed something which they call the demographic transition.

The demographic transition is a shift of birth and death rates from high levels to low levels in a population.

And that usually is a result of economic and social development. You have several stages. Typically we speak of four stages.

We have the first stage as high birth and high death rates where there is slow population growth.

Then we have the second stage where we have high birth rate and falling death rates because of improvements in food supplies,

sanitation technology, basic health care, education, etc. And that's where we have high growth rates.

Then stage three, declining birth rates and relatively low death rates. And this is characterized by slow growth rate of the population.

The growth begins to level off. And then stage four is where you have low birth rates and low death rates and consequently a low population growth.

So as countries get richer, their fertility rates drop. And it's long been assumed that that's the way it will continue to go.

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