The ratio of births observed to births needed: An indicator to assess demographic sustainability

Background: A stationary population may offer advantages for societal, economic, and ecological sustainability. It requires that the number of births aligns with expected deaths under given mortality patterns. However, reproductive rates such as the TFR are not designed to assess whether the number of births observed is sufficient to replace expected deaths, necessitating direct assessment of birth–death balance for population sustainability. Objective: We aim to assess whether current fertility levels are sufficient to replace expected deaths using a new demographic indicator, the ratio of births observed to births needed (Bo/Bn). Methods: We analyze global and regional demographic sustainability from 1950 to 2100 using data from the World Population Prospects 2024. The Bo/Bn ratio is derived from stationary population theory, comparing actual births to those required under given mortality conditions. Results: Globally, births observed exceeded those needed by 18% in 2025, but this surplus is declining. According to the United Nations medium variant projection, by 2060, global births will be insufficient to replace expected deaths for the first time in modern history. By 2100, births are projected to fall 12% short of the level needed. Regional variation is substantial: Middle and Eastern Africa maintain Bo/Bn ratios above 2.0, while Eastern Asia exhibits a critically low ratio of 0.49. Currently, 43% of the global population live in countries with insufficient births, and this proportion will reach 75% by 2100. Contribution: The Bo/Bn ratio provides a direct measure of demographic sustainability that complements traditional fertility indicators, offering crucial insights for policy planning in an era of global demographic transition.