The UN recently announced that the world’s population reached the 7 billion mark on 31 October 2011 and confusingly several babies have been symbolically chosen to represent this milestone by different organizations and by individual states.

Figure 1. Which baby really is the 7 billionth?

There has also been some debate about the accuracy of the estimated date. The UN population estimates chief, Gerhard Heilig, suggested a 1-2% margin of error, which means that the 7 billionth person could be born 6 months either side of this date. However, the UN wanted to highlight the milestone of having 7 billion people on the planet given the potential impact on the planet and global resources.

If you want to get a (rough) estimate of where you come in the global family check out this tool from the BBC website.

Figure 2. Where do you fit in the global family?

As you can see from Figure 2. the global population has increased fast – especially over the last century. There are three main components of population change: (1) Fertility, (2) Life expectancy i.e. mortality and (3) Net migration. The many contributing factors to the recent population surge include vaccination, improved medical care and living conditions and increasing food supply (due to pesticides and GM foods). As countries become more industrialized the resulting longer lifespan is balanced by declining birth and fertility rates.

Birth rate and fertility rate – what’s the difference? The birth rate is the number of children born per 1,000 people per year. It contributes directly to population changes in a given year. The total fertility rate is defined as the average number of children born to each woman over the course of her life, in other words how many children the average woman will have during her lifetime. Because total fertility rates look at the birth rate across different age groups it is more informative than the birth rate. More developed countries tend to have lower birth and fertility rates, which are driven by factors such as female education, gender equality, contraception and family planning and national policies (UNFPA State of World Population 2011).

Global trends in fertility rate show that worldwide there is a decrease in the total fertility rate (Figure 3) even in the world’s least developed countries (30%). This means that in these countries women are having 30% fewer children on average. The less developed countries are catching up even more quickly with the more developed countries in terms of fertility rate (56% decrease compared to 29% decrease in developed countries).

Figure 3. Global Fertility rates (data extracted from UN World Fertility Patterns 2009). Note that “Less developed” category excludes the least developed countries as set out in this document.

So if total fertility rates are decreasing how is the world population still increasing so quickly? The answer lies in the replacement rate, i.e. the total fertility rate required to sustain the population at its current level. To sustain a population at a given level in mathematical terms the following must be true:


We can ignore migration effects at the global level (at least until we begin inhabiting other planets!) – so the birth and death rates must balance for a constant population. Assuming also that the mortality rate remains unchanged, in order to sustain the population, each woman must have two children over her lifetime to replace both herself and her partner. In reality the replacement rate is slightly greater than two to account for infant mortality and other factors. From Figure 3 we see that the world total fertility rate is greater than 2 (2.6 between 2005-2010). So although the total fertility rate has dropped dramatically compared to that in 1970-1975, it is still a contributing factor to the rising global population. The graph also implies that the global total fertility rate is driven by the less and least developed countries.

Figure 4 shows the total fertility trends in European Union countries (though not all countries were EU member states at the time of these analyses). While in 1970 there were differences in the fertility rates (lowest in Finland (1.8) and highest in Ireland (3.9)) the fertility rates have evened out more recently (1.2 to 2.0). EU total fertility rates have been consistently below the world average. Furthermore, the total fertility rate for all of these countries is now below the approximate replacement rate of 2.1. Therefore, population increases in the EU are largely driven by net inward migration (and also possibly increase in lifespan – though this was not looked at closely).

Figure 4. European Union Fertility rates (data extracted from UN World Fertility Patterns 2009), Note that a Replacement Rate of 2.1 was assumed for developed nations. Not all countries were part of EU at the time of this analyses.

So what’s the pattern in the UK? The total fertility rate has dropped from an average of 2.4 in 1970-1975 (possibly a hangover from the babyboom years) to 1.8 in 2005-2010. The population, however, is increasing steadily and is projected to rise, as shown in Figure 5.

Figure 5. ONS 2010-based national population projections

Given what we know about the replacement rate, the other two main factors – i.e. net migration and life expectancy -must be driving the increase in the population, a picture mirrored at the European level.

As the world population continues to rise, the trends in the developed world may be an indicator of a slowdown in global population growth. However, we are unlikely to see that trend for some time. Extrapolating from Figure 2 we can see that the world’s population is predicted to reach 10 billion soon after 2050. So the question is, by then will we be able to predict who the 10 billionth baby really is?


Baby articles:

BBC Article on accuracy of population numbers:

UNFPA paper:

UN data:

ONS 2010 population projection:

General Reading: