Abstract
Introduction
Fertility is declining globally. The fertility rates in more than half of all countries and territories in 2021 have declined below replacement level. 1 This decline in global fertility is thought to be multifactorial in nature, that is, explained by economic and behavioral developments, but also by biological and environmental factors, such as the exposure to chemicals that are originating directly or indirectly from fossil fuels. 2 The aim of this commentary is to provide an overview of recent demographic studies and forecasts on the state of fertility and to highlight the relevance of current and future fertility trends for reproductive endocrinology and infertility providers.
Overview of the Global Burden of Diseases, Injuries, and Risk Factors Study 2021 Results and Findings
The assessment of major demographic indicators of fertility and the creation of forecasts thereof are crucial for the understanding of current and future social, economic, and geopolitical developments. One important source of such an assessment is the 2022 Revision of World Population Prospects (WPP 2022), which is produced by the Population Division of the UN Department of Economic and Social Affairs. 3 However, the WPP 2022 was questioned as to its adherence to the Guidelines for Accurate and Transparent Health Estimates Reporting statement and for incorrectly projecting gradual increments in the total fertility rate (TFR) in countries with fertility rates that are stagnant. The TFR estimates the mean number of children a woman will have during her reproductive years in a defined geographic area. A TFR of 2.1 is generally considered replacement-level fertility, that is, one resulting in neither a decrease nor an increase in the population in the territory under assessment.
The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2021 generated comprehensive demographic assessments of key fertility indicators at the global, regional, and national levels from 1950 through 2021. In addition, the GBD 2021 projected fertility metrics through 2100 based on a reference scenario and key alternative scenarios related to the successful implementation of global education and family planning scenarios. 1 For GBD 2021, mixed-effects regression models and spatiotemporal Gaussian process regression were used to synthesize data from 8709 country-years of vital and sample registrations, 1455 surveys and censuses, and 150 other sources, with an eye toward generating age-specific fertility rates (ASFRs) for 5-year age groups from 10 to 54 years of age. ASFRs were then tallied across age groups to produce TFR estimates.
The GBD 2021 Fertility and Forecasting Collaborators found that from 1950 to 2021, the global TFR more than halved, from 4.84 (95% uncertainty interval = UI 4.63–5.06) to 2.23 (2.09–2.38). The total number of live births was at its highest in 2016 at 142 million, decreasing to 129 million in 2021. Since 1950, all territories and countries of the world have seen declines in fertility rates, with a TFR above the generally accepted replacement level of 2.1 in 94 countries and territories (46.1%) in 2021. Almost all countries (44/36) in sub-Saharan Africa featured a TFR >2.1, thereby making this region the one with the largest share of live births in 2021. The global TFR was projected to decline to 1.83 in 2050 and to 1.59 in 2100 based on the GBD 2021 reference scenario, with fewer and fewer countries exceeding replacement TFR levels. According to the reference scenario, 49 countries and territories (24.0%) will feature a TFR >2.1 in 2050, with a further decrease to only 6 (2.9%) in 2100 in Samoa, Somalia, Tonga, Niger, Chad, and Tajikistan. Three of these countries are in sub-Saharan Africa, wherein more than half of the world’s live births (54.3%) will take place in 2100, according to the reference projection. 1
Additional GBD 2021 forecasts in each location were based on the achievement of the UN Sustainable Development Goals (SDGs) for education by 2030; the achievement of the SDG contraceptive met need target by 2030; the enactment of pro-natal policies to create supportive environments for those who give birth; and a combination of these three scenarios. Forecast estimates for the combined achievement of SDG targets for education and contraceptive prevalence met need resulted in global TFR forecasts of 1.65 in 2050 and 1.62 in 2100.
Dynamics of fertility decline in modern societies
The classical and well-established concept of fertility transition postulates that societies shift from a state of high fertility and high mortality to a state of low fertility and low mortality with an aging population. 4 Mass education and a fully developed system of capitalist production are drivers of this transition as well as determinants of the timing of fertility decline in a given society. 5 It has been observed that the fertility decline in European regions resulted from a gradual diffusion of new fertility behavior from the French-speaking regions of Europe to the rest of the continent. 6 Low levels of fertility—below replacement level—ultimately result in inverted population pyramids, replete with declining numbers of people in the working-age population segment. There appears to be an inverse relationship between fertility and the level of development of a country or a territory. As more and more countries become increasingly developed, human civilization appears to be rapidly heading toward a long-term low-fertility status quo, except for the low-income regions in sub-Saharan Africa which are projected to sustain comparatively high fertility rates.
Economic and societal consequences of different fertility rates in higher-income countries compared to poorer regions of the world
High-income countries typically consume disproportionately more resources—food, water, and energy than low-income counterparts. The impact of declining fertility and population size in high-income countries with sustained or rising fertility in low-income countries is challenging to predict. On the one hand, a declining population diminishes the economic productivity of a society, which perhaps favors a leveling of the economic playing field. On the other hand, the lack of proportionality in resource consumption relative to population size raises the possibility that the shrinking population will further concentrate wealth and resource utilization, thereby fueling a perpetually widening gap between the high- and low-income countries.
Environmental factors in the decline in global fertility
An increasing body of evidence suggests that, in addition to economic and behavioral factors, environmental factors play a key role in the global fertility decline. 2 In this context, an important distinction needs to be made between the demographic meaning of fertility (defined as the number of children per reproductive-age female [TFR]) and the biological meaning of fertility (the ability to reproduce, defined and referred to as fecundity). While socioeconomic and cultural factors undoubtedly play a major role in the global fertility decline, biological causes, including lifestyle factors and environmental exposures, are also likely involved. Disentangling these differential effects represents a major challenge to research in this area. 7
Forecast for assisted reproductive technology usage in developed and developing countries
As a society evolves, fertility declines, and the mean age at first birth increases. Consequently, there is a rising relative and absolute usage of medically assisted reproduction (MAR), including assisted reproductive technology (ART), in industrial regions. In Denmark, for example, 1 in 10 newborn babies are now conceived by MAR. 8 Usage of MAR, including ART, could thus be viewed as a marker of societal development.
Consequences of increased use of ART in developed countries
It has been hypothesized that the increased use of ART will result in a rising need for ART in future generations, as current infertility may beget future infertility. As an example, in the treatment of male factor infertility, genetic conditions that were previously incompatible with conception, are now treatable, with the possible consequence of perpetuating genetic traits and thus the predisposition of future generations to infertility.9,10 Given the high likelihood that similar mechanisms are at play for female factor infertility, it can be prognosticated that the increasing use of ART will amplify the need for the future need for ART, in addition to the phenomenon of global fertility decline. 11
Take-home messages and future directions
Fertility rates across the globe are declining. By 2100, fertility rates will be below replacement level in over 95% of countries and territories worldwide. Societal, cultural, and biological factors are responsible for this phenomenon. Declining fertility rates may be viewed as a marker of societal development, in that the largest relative contribution of live births globally will come from low-income settings, particularly a subset of countries and territories in sub-Saharan Africa. In the face of declining fertility overall, the need for and usage of MAR including ART will increase worldwide, a phenomenon which will further perpetuate the incidence of infertility in future generations. National and international policy makers and infertility providers should be aware of these global health phenomena so as to guide strategy development and resource allocation in the provision of fertility services.
Footnotes
Author Disclosure Statement
No competing financial interests exist.
Funding Information
No funding was received for this commentary.