Reproductive life concentrated into ever-shorter windows*

Demographers have long tracked how many children women have and when. But how widely fertility is spread across reproductive ages has received comparatively little attention. Thomas Spoorenberg introduces a simple percentile-based measure to fill this gap, the childbearing span, and applies it to seven decades of global data.

Across most of the world, women today are having fewer children than their mothers or grandmothers did. This much is well known. Less examined is how the timing of those births has shifted: not just when women start having children, but how compressed or spread-out childbearing has become across the reproductive years.

Since 1950, the median total fertility rate has fallen from nearly six births per woman to under two (United Nations 2024). As fertility levels converged worldwide, so too did the analytical toolkit used to monitor them: the total fertility rate (TFR) summarizes the quantum of fertility, and the mean age at childbearing (MAC) captures its central timing. Both indicators are indispensable, but both reduce a complex distributional process to a single number. They cannot reveal whether births cluster tightly around the mean age or spread across the full reproductive lifespan. Two countries can share identical TFR and MAC values yet exhibit very different fertility schedules: one concentrated between ages 25 and 35, another stretching from adolescence into the forties.

What existing approaches do and don’t capture

Efforts to go beyond averages have a long history in demography. An influential line of research quantifies how concentrated reproduction is among women, i.e. what share of women account for what share of all births. Drawing on Lorenz-curve principles, Vaupel and Goodwin (1987) constructed such indices for the United States, and Lutz (1989) extended them globally; Shkolnikov et al. (2007) later applied similar tools to European and American cohorts. A separate strand examines the shape of the fertility age schedule through its mean and variance, though these parameters assume a symmetric distribution and struggle when fertility schedules are skewed or multi-peaked (Chandola et al. 1999). More recently, percentile-based metrics have proven their value in measuring inequality in ages at death (Shi et al. 2022), pointing toward an approach that is robust to irregular distributions and straightforward to interpret.

What has been missing is a measure focused on the temporal spread of fertility by age: not who bears children, but over how many reproductive years most births occur in a given population. This is the gap the childbearing span is designed to fill (Spoorenberg 2026). It offers a direct and globally comparable lens for tracking this distributional dimension of fertility.

The childbearing span: concept and intuition

The childbearing span measures the number of years separating the age at which a population reaches 10% of its total fertility from the age at which it reaches 90%. It captures the width of the fertility schedule (i.e. how many years of reproductive life account for the bulk of childbearing) rather than simply its level or central timing. A span of 13 years, as observed in India today, means that 80% of births are concentrated into just 13 years. A span of 19 years, as in Peru, reflects a much more spread-out pattern. Both countries share nearly identical total fertility rates, yet their reproductive age distributions look strikingly different.

This distinction matters for several reasons. The spread of fertility affects demographic momentum, the overlap of generations, and intergenerational support structures. It is also closely tied to the social transformations, such as in education, employment, contraception, and gender norms, that reshape when and how reproduction is organized within societies.

A worldwide compression, unfolding unevenly

Applying this approach to a global dataset from 1950 to 2025 reveals a clear global trend: childbearing has been compressed into steadily shorter reproductive windows over the past 75 years. The global average childbearing span declined from about 19 years in 1950 to under 17 years in 2025, meaning the world’s reproductive age distributions have narrowed by roughly two years on average (Figure 1).

Behind this overall trend lie sharply divergent regional trajectories. Eastern and South-Eastern Asia underwent the most dramatic compression: spans fell from around 19.5 years in 1950 to approximately 13.5 years by the early 1990s, driven by rapid fertility transitions, strong government policies, and normative convergence around socially sanctioned ages for childbearing. Today, this region stands at the extreme end of concentration globally.

Sub-Saharan Africa tells a different story. Despite substantial fertility decline from over six births per woman to around 4.3, the region’s childbearing spans have remained consistently broad, barely narrowing over seven decades. This persistence suggests that wide age distributions are maintained by structural and cultural factors such as the continued social importance of motherhood, plural marriage systems that extend fertility into older ages and, possibly, risk-diversification strategies in high-mortality and high-uncertainty environments.

Europe, Northern America, Australia, and New Zealand display yet another pattern. After early compression through the 1970s, spans temporarily widened between 1980 and 2010, when fertility postponement intensified, before narrowing again. This U-shaped trajectory illustrates how period measures are sensitive to ongoing timing shifts: when large numbers of women delay births simultaneously, fertility appears more dispersed in any given year than it actually is across cohorts.

Concentration is not simply a function of fertility level

One of the most striking findings concerns the relationship between fertility levels and concentration patterns (Figure 2). In 1950, despite vast differences in how many children women had, the width of fertility schedules was remarkably similar across countries: most childbearing spanned an interval of between 45% and 55% of the 35-year reproductive period. By 2025, that uniformity has dissolved. Countries with identical total fertility rates of around two births per woman now display concentration shares ranging from 30% to 50%, representing a near two-fold difference in the proportion of reproductive life devoted to childbearing.

This decoupling of level from distribution signals that fertility concentration has become an independent demographic dimension, shaped by cultural norms, institutional contexts, and socioeconomic structures that operate separately from the quantum of fertility. Understanding why countries at similar fertility levels have such different age distributions requires looking beyond the number of children to the social organization of reproduction itself.

A simple tool with broad analytical reach

The childbearing span requires only standard age-specific fertility rates (i.e. data that are relatively easily available) and yields an immediately interpretable quantity: a number of years. It can be disaggregated by region, income group, or social subpopulation whenever data allow, and it can complement the total fertility rate and mean age at childbearing in routine demographic monitoring.

As fertility continues to decline globally, differences in concentration patterns may become increasingly consequential, reflecting distinct experiences of reproductive timing across populations and raising questions about equity, autonomy, and the conditions under which women make childbearing decisions. The childbearing span offers a straightforward entry point for tracking these distributional dynamics across time and place.

* The views expressed herein are those of the author and do not necessarily reflect the views of the United Nations.

Reference

Chandola, T., Coleman, D.A. and Hiorns, R.W. 1999. “Recent European fertility patterns: Fitting curves to ‘distorted’ distributions.” Population Studies 53(3): 317–329.

Lutz, W. 1989. Distributional Aspects of Human Fertility: A Global Comparative Study. Academic Press.

Shi, J., Aburto, J., Martikainen, P., Tarkiainen, L. and van Raalte, A. 2022. “A distributional approach to measuring lifespan stratification.” Population Studies 77(1): 15–33.

Shkolnikov, V.M., Andreev, E.M., Houle, R. and Vaupel, J.W. 2007. “The concentration of reproduction in cohorts of women in Europe and the United States.” Population and Development Review 33(1): 67–100.

Spoorenberg, T. 2026. “Global trends in the compression of childbearing spans, 1950–2025: A new approach to the age distribution of fertility.” Journal of Population Research 43:12. https://doi.org/10.1007/s12546-026-09420-6 

United Nations. 2024. World Population Prospects 2024. Department of Economic and Social Affairs, Population Division. https://population.un.org/wpp/ 

Vaupel, J.W. and Goodwin, D.G. 1987. “The concentration of reproduction among U.S. women, 1917-1980.” Population and Development Review 13(4): 723–730.