Another Effect of Climate Change: A Warming Climate Impacts Pregnancies

The effects of global warming on fertility were recently examined in an international study published in the journal Population Studies. According to the results, fertility rates in Spain declined nine months after periods of extreme heat. This phenomenon has not only been investigated at the European level. Tamás Hajdu, a researcher at the HUN-REN Centre for Economic and Regional Studies, calculated conception rates in Hungary based on all births, abortions, and foetal deaths since 1980, measured as the number of conceptions per thousand women aged 15–44. He then examined how these rates correlated with temperature.

The researcher described the impact over time as a U-shape: the number of conceptions falls most sharply two to four weeks after a heatwave. For example, three weeks after a week of extreme weather, where the average daily temperature exceeds 25°C (considered a heatwave day), the number of conceptions is almost six percent lower than following a week with average daily temperatures between 15°C and 20°C.

He stressed that warm weather is unlikely to affect the number of conceptions by changing sexual activity; rather, it primarily affects the fertility potential of both men and women. He also emphasised that hot weather does not make these pregnancies 'disappear' but instead delays a significant proportion of them. In fact, conception rates rise four to six months after a week of high temperatures.

Not only the number of pregnancies is falling

Heatwaves not only reduce the likelihood of conception but also affect the outcome of pregnancies that do occur. Estimates suggest that 40–70 per cent of conceptions do not result in live births, and the vast majority of these foetal losses occur in the first few weeks of pregnancy, before clinical detection.

Thus, if the weather is unusually warm during the first few weeks of pregnancy, the number of early miscarriages increases (while it decreases in cold weather). The researcher also demonstrated that when the weather gets warmer during pregnancy, the average birth weight of newborns decreases.

In figures, this means that a day with temperatures between 20 and 25°C reduces birth weight by 0.36 grams, while a day above 25°C reduces it by 0.46 grams. This may seem minor, but over the course of a 39-week pregnancy, these effects accumulate and become more substantial, Tamás Hajdu highlighted.

Pregnancies are also affected

Although the overall change in the number of births is small, some reordering within the year is expected. Fewer pregnancies will start during the summer months, but most will not "disappear"; they will simply be delayed, meaning that more conceptions are expected in late autumn and winter. "However, these effects are also moderate, and we can expect a few percentage points of reordering," Tamás Hajdu explained. He also noted that although climate change is not the primary factor influencing birth rates, it is an aspect that is rarely considered when discussing climate change, even though pregnancies will not remain unaffected by a warming climate.

The results show that temperature has a measurable and non-negligible effect on human pregnancies, although the impact of many social, behavioural, and environmental factors may be much stronger than that of the weather. Furthermore, the temperature increase caused by climate change in Hungary raises the risk of miscarriages and negatively impacts the health of newborns at birth.