How did this happen, and can this trend reverse?

To answer that question, Visual Capitalist's Pallavi Rao tracks the total fertility rate of every European country per UN estimates for 2025.

The fertility rate measures the average # of children a woman in a specific area will have over the course of her lifetime.

From this number, demographers and statisticians can make assumptions about population replacement and future trends.

Ranked: European Countries by Total Fertility Rates 2025

As it happens, 2.1 is a key threshold for fertility rates.

At this level, an area's population remains stable. Above it, it's likely to grow (barring war, disease, and famine). And below it, it's likely to decline (barring immigration).