Heat and fertility in Europe: hotter summers linked to fewer births in France and Italy

Two new studies link hot weather to reduced births in France and Italy, showing roughly 360 fewer births in France and 192 fewer in Italy on days when average temperatures reach or exceed 25°C. The impact surfaces nine to ten months after exposure, implying fewer conceptions during warm months. Researchers discuss two plausible mechanisms for this link and caution that limitations of population data may affect interpretation. Original publisher: Nature.

• Each warm day (≥25°C) corresponds to about 360 fewer births in France and 192 fewer births in Italy
• Effect emerges nine to ten months after exposure, suggesting fewer conceptions
• Heat may harm reproductive health in both men and women and possibly alter sexual behavior
• Findings align with broader European and US studies, hinting at a regional pattern

Overview

Recent studies have identified a relationship between hot weather and lower birth counts in Europe. In two analyses focused on France and Italy, researchers found that each day with an average temperature of 25°C or higher was linked to a substantial drop in births — roughly 360 fewer births in France and about 192 fewer in Italy. The timing of the effect is nine to ten months after exposure, which aligns with a reduction in conceptions during warm months rather than a shift in birth timing alone. The studies consider normal annual patterns in births and temperatures and account for regional and national trends, but they rely on population-level data rather than controlled experimental conditions.

Key findings

The central result is the association between hot days and reduced births, with a lag of several months between exposure and the observed change in birth counts. This lag corresponds to the typical human gestation period, suggesting that hot weather may depress conceptions more than it delays births. The magnitude of the effects is estimated to be fairly small in the context of overall fertility declines but could become more consequential if heat becomes more frequent or intense in an ageing, low-fertility Europe.

Possible mechanisms

The authors propose two broad channels through which heat could influence fertility. First, heat exposure can impair reproductive health in both men and women, potentially reducing conception likelihood. Second, hot weather could influence sexual behavior, leading to fewer pregnancies. Either pathway or a combination could explain the observed declines. The studies emphasize that, while the association exists, it is not the dominant driver of Europe’s fertility trends, which are also shaped by economic conditions, family norms, and policy environments.

Context and comparisons

France and Italy are not alone in showing this pattern. Earlier research in the US, Spain, Hungary, and across Europe has reported links between higher temperatures and lower birth rates. Some earlier US studies noted that missing births were later compensated by births in subsequent months, but the French and Italian work finds no sign of this catch-up, implying that some births might be lost for good in these contexts.

Geopolitical and demographic implications

With Europe’s fertility already low and populations ageing rapidly, each lost birth has amplified consequences for demographic structure and social policy. The Mediterranean region is warming particularly quickly, and Europe as a whole is experiencing accelerated warming. If hot weather becomes more frequent, climate change could become a more significant factor in fertility trends and related population policies.

Limitations and future directions

The studies use observational data and attempt to adjust for seasonality and long-term trends, but causality remains challenging to establish. The authors acknowledge that weather and fertility dynamics may change as climates continue to evolve, and they caution that norms, economic pressures, and policy contexts will continue to shape fertility in the years ahead.