UN data suggests that the world’s population will hit 11 billion by 2100, with the fastest rises being recorded in Africa and Asia.

How many people are there in the world?

We don’t know for sure as all figures are estimates, but UN data suggests there were about a billion people in 1800, 2 billion in 1927, 5 billion in 1987 and just over 7.5 billion today.

There are on average about 250 babies born every minute – more than 130 million in a year. It is projected that there will be 11 billion people by 2100. New UN figures are due out in June.

Most national governments make their own population projections. The United Nations and the World Bank figures are the most widely used globally.

Since the 1960s, more boys than girls have been born every year. About 117 million women are believed to be “missing” in Asia and eastern Europe – due to discriminatory son preference and gender-biased sex selection.

Over the last 30 years, some regions have seen up to 25% more male births than female births, reflecting the persistent low status of women and girls. The consequent gender imbalance can have damaging social effects such as increased sexual violence and trafficking.

Where is the population rising fastest – and slowest?

Broadly speaking, the fastest population rises are being recorded in Africa and Asia, which will have 15 of the 20 most populous nations by 2050. By that year, there will be more Nigerians than Americans. By 2100, it is projected that as many as one-third of all people – almost 4 billion – will be African.

At the other end of the scale, population growth has stalled – or even gone into reverse – in parts of western Europe, Japan and Russia.

Are these numbers sustainable?

A nuanced question. Experts like Paul Ehrlich argue that the population of the world has long since surpassed optimal levels, though critics counter that consumption is as important as population levels.

Some believe the very argument about overpopulation is controversial as it tends to point the finger at poorer parts of the world with large growth rates, and not at richer regions, which use disproportionately high amounts of resources.

What influences fertility?

The fertility rate is the number of children born for every woman of childbearing age in a population. The things that tend to affect it include female empowerment, wellbeing and the status of children, technological and economic changes, and opportunities for family planning.

The level of education in a society – of women in particular – is one of the most important predictors for the number of children families have.

The global average fertility rate is just below 2.5 children per woman today. Over the last 50 years the global fertility rate has halved, as some of these factors bore down on family sizes.

In the pre-modern era, fertility rates of 4.5 to 7 children per woman were common. At that time, high mortality rates of young people kept population growth low. As health improved, the population growth rate began to soar, only flattening out as the fertility rate declined towards 2 children per woman.

A record number of women now use contraception. Figures from the UN’s Department of Economic and Social Affairs show 64% of married and cohabiting women used modern or traditional methods of contraception in 2015 – a significant rise from 36% in 1970. But the figures show wide disparities between and within regions and countries.

Africa has the lowest percentage of women using contraceptives, and the highest unmet need in the world. Despite this, some African countries have made the biggest leaps in contraception use over the past 40 years and are projected to make the greatest gains in the next 15.

In Mauritius, rapid population growth in the early 60s led the government to launch a family planning programme, and the country now boasts the highest rates (75.5%) on the continent.

If birthrates have fallen so far, why is the population still rising fast?

Of course, fertility rates are just half the story. People are living longer – far longer in some parts of the world. About 55m people die every year, which is less than half the number who are born.

The number of children who die before reaching their fifth birthday has fallen to an all-time low: it is currently less than half what it was in 1990.

A child’s chance of survival is still vastly different depending on where they are born.

Sub-Saharan Africa has the highest under-five mortality rate – 79 deaths for every 1,000 births, which means that one child in 13 die before their fifth birthday. This compares with six for every 1,000 in Europe and northern America and four for every 1,000 in Australia and New Zealand.

At the same time, life expectancy is higher than 80 in 30 countries and higher than 70 in more than 100 countries.

So what is the demographic dividend?

Countries that do succeed in reducing fertility rates can benefit from a demographic dividend, where there are more people in work than children to support.

Where you have a rapid decline in fertility, the younger population is no longer growing as fast and the economy should receive a boost because the number of workers per child increases, and that should provide a period of rapid economic growth. This was the experience in the east Asian “tiger” countries like South Korea and Taiwan in the 70s. Now countries such as China and India are benefiting from a demographic dividend.

A consequence of falling child mortality but continuing high fertility is a “youth bulge” – a high population of young people. In Africa, because rising numbers of increasingly educated people have not been matched by jobs, this has led to significant youth unemployment.

A young population offers a lot of potential for the development of a country, but only if their talents are realised through investment. If there is little support given to young people to develop the skills they need as they enter the labour market, then the economy misses out.

Isn’t it problematic that western populations are declining?

Another global demographic shift is ageing populations in developed countries such as Japan and Germany, and also in advanced developing countries. In nations as diverse as Bangladesh, Cambodia, Mongolia and Vietnam, the population aged over 60 will triple by 2050.

This year, the number of people worldwide who are over 60 will rise above 1 billion for the first time. By 2050, it is forecast to be 2 billion. This raises the question: who will pay for them?

Falling birth rates can mean fewer young workers entering the labour force at a time when the healthcare and social support costs associated with ageing are likely to rise.

But ageing populations can be a cause for celebration. It means development has taken place.

If countries plan for the shift they can see gains. In Japan, for example, the introduction of universal health coverage meant more treatment for high blood pressure, and therefore fewer strokes, extending worker productivity.

If countries consider redesigning pension systems and offering flexible work in retirement, seeing older volunteers as a potential resource, then many more mature members of society can have not just greater life expectancy but also be healthier for longer.

Later retirement does not have to mean fewer jobs for the young. More older people working can increase GDP and generate more demand for young workers.

What next?

The funding battles over international family planning are ongoing. US Congress rejected the 2018 budget. The debates will continue in 2019.

Family planning organisations are learning that to survive political shifts and budget cuts, they need to diversify their sources of funding. This means seeing family planning as not just a public health concern but also about development and a clear return on investment.

Where women have control over their own fertility, there are gains well beyond their own families.