The fertility rate of a country is the average number of children that women from that country will have throughout their reproductive years. From 1840 until 1910, Russia's fertility rate was relatively consistent, remaining between 6.7 and 7.4 children per woman during this time. Between 1910 and 1920, the fertility rate drops sharply as a result of the First World War and Russian Revolution (for individual years of WWI, the fertility rate dropped as low as 3.4). From 1920 to 1930 the fertility rate returns above 6 again, however a gradual decline then begins, and by the end of the Second World War, the Russian segment of the Soviet Union's fertility rate was below 2. The population experienced a relatively small 'baby boom' in the two decades following the war, but then the fertility rate dropped again, most sharply between 1990 and 1995 at the end of the Soviet Union's reign. Russia's fertility rate reached its lowest point in 2000 when it fell to just 1.25 children per woman, but in the past two decades it has risen again, and is expected to reach 1.8 in 2020.

While the BRICS countries are grouped together in terms of economic development, demographic progress varies across these five countries. In 2019, India and South Africa were the only BRICS countries with a fertility rate above replacement level (2.1 births per woman). Fertility rates since 2000 show that fertility in China and Russia has either fluctuated or remained fairly steady, as these two countries are at a later stage of the demographic transition than the other three, while Brazil has reached this stage more recently. Fertility rates in India are following a similar trend to Brazil, while South Africa's rate is progressing at a much slower pace. Demographic development is inextricably linked with economic growth; for example, as fertility rates drop, female participation in the workforce increases, as does the average age, which then leads to higher productivity and a more profitable domestic market.

In 2024, Monaco was the European country estimated to have the highest fertility rate. The country had a fertility rate of 2.1 children per woman. Other small countries such as Gibraltar or Montenegro also came towards the top of the list for 2024, while the large country with the highest fertility rate was France, with 1.64 children per woman. On the other hand, Ukraine had the lowest fertility rate, averaging around one child per woman.

In 1800, the population of the modern-day territory of Iran was approximately 6.3 million. This figure would see modest growth throughout the 19th century, as several wars and a mass famine in 1870-1871 (modern estimates put its death toll at around 1.5 million people) were largely balanced out by a surge in migration to Iran; this migration came as the Russian Empire expanded into the Caucuses, and caused a wave of refugees to flee southwards to avoid forced expulsion and ethnic cleansing in the North Caucasus region, particularly from 1864 onwards. As a result, the population of Iran reached ten million by the turn of the 20th century.

Twentieth century growth Iran’s population would begin to grow rapidly in the 20th century, as the discovery of oil in the country in 1908 led to an economic boom, and the socio-economic reforms implemented under Reza Shah would see a number of medical and healthcare advancements across the country. Although unpopular with religious fundamentalists, Reza Shah's reforms had long-term influence on the demographic development of Iran, even after his abdication in 1941. Following the Second World War, Iran became increasingly westernized and developed relatively strong relations with the U.S.; however, western influence, economic imbalances and the oppression of the Mohammed Reza Shah's regime became the driving forces behind the Iranian Revolution, which was one of the most significant moments in the history of the region.

Growth after the Revolution The 1979 Iranian Revolution saw the removal of the Shah and an end to Iran's so called westernization; the monarchy was replaced by an Islamic, theocratic regime led by the Supreme Leader, Ayatollah Ruhollah Khomeini. During Khomeini's decade in charge he oversaw Iran's transition into an Islamic Republic, which implemented radical political and cultural changes in the country, and this coincided with an increased population growth rate in the 1980s. This growth was promoted by the Iranian government, who encouraged a baby boom during the Iran–Iraq War between 1980 and 1988, as part of an effort to increase future Iranian military manpower. As a result of this strategy, the population of Iran would grow from approximately 38.6 million in 1980 to over 56 million just a decade later. Following the implementation of a UN-brokered ceasefire in 1988, population growth in Iran would slow, as economic sanctions and government implementation of family planning policies would lead to a drop in fertility. Population growth has continued steadily into the 21st century, however, and in 2020, Iran is estimated to have a population of 84 million.