Higher Temperatures During Pregnancy Could Affect Fetal Growth

The consequences of climate change are usually associated with melting glaciers, increasing extreme weather events, or more frequent heat waves. But rising temperatures can also have an impact on human health—even before birth. A recent study by researchers at the Medical University of Vienna, the University of Vienna, and the University of Calgary now provides evidence that higher ambient temperatures during pregnancy may be linked to changes in the growth of unborn children.

The scientists investigated whether the temperature to which pregnant women are exposed during various stages of pregnancy has an impact on the physical development of the fetus. The result: Higher temperatures during specific windows of pregnancy are associated with shorter body length in newborns. The findings were recently published in the American Journal of Human Biology and draw attention to a previously overlooked aspect of climate impact research.

More than Three Decades of Birth Data Analyzed

Although the observed differences in birth length are relatively small, the researchers see this as an important indication that environmental factors such as heat can influence biological development processes even in the womb. Given rising global temperatures, this field of research is becoming increasingly important. For their study, the research team drew on an exceptionally extensive dataset. The data analyzed included more than 1.2 million singleton births in Austria registered between 1984 and 2014. The study thus spans a period of three decades and provides a comprehensive view of long-term trends.

The birth data were combined with detailed meteorological records from a total of 39 Austrian districts. This enabled the researchers to reconstruct the average ambient temperatures to which the mothers were exposed during various stages of their pregnancies. A particular strength of the study lies in its scale. While many studies on pregnancy and environmental factors are limited to a few thousand participants, the analysis of more than one million births provides significantly greater statistical power. Even very small effects can thus be detected.

In addition, the research team took into account numerous other factors that can influence a child’s growth in the womb. These included, among others, the mother’s age, her level of education, the duration of pregnancy, the child’s gender, the number of previous births, the year of birth, and the mother’s ethnic background. The inclusion of these variables was intended to ensure that the observed differences could be attributed as precisely as possible to temperature exposure.

Pregnancy as a Sensitive Developmental Phase

The development of a child in the womb proceeds in several phases, each characterized by different biological processes. Even small changes in environmental conditions can have a greater impact at certain developmental stages than at others. In the first weeks after conception, the body’s basic structures are formed. Organs, the nervous system, and various tissues begin to develop. This phase has long been considered particularly sensitive to external influences such as pollutants, medications, or health issues affecting the mother.

Later, the focus of development shifts increasingly toward growth and maturation. The unborn child gains weight and size, while the organs continue to develop their functions. Against this backdrop, the researchers were particularly interested in whether there are periods during pregnancy in which temperature stress exerts a stronger influence on fetal growth than in other phases.

Critical Time Windows for the Influence of Heat

The study’s results show that temperature is not equally correlated with birth length throughout the entire pregnancy. Instead, the researchers were able to identify two particularly sensitive time windows. A clear correlation was already evident in the first weeks after conception. Higher temperatures during this early phase were associated with shorter birth length. This suggests that early embryonic development may be sensitive to environmental conditions.

A second sensitive window began around the 15th week of pregnancy and lasted until birth. It is precisely during this phase that a significant portion of physical growth takes place. The fetus continues to develop rapidly and grows steadily in size. The results suggest that higher temperatures during this period could have a slight effect on growth. Interestingly, the researchers found no statistically significant association between temperature and birth length for the middle phase of pregnancy—roughly between the 10th and 14th weeks of pregnancy. Why this period appears to be less sensitive to heat remains unclear and could be the subject of future studies.

How Significant is the Observed Effect?

At first glance, the influence of temperature on birth length appears small. The researchers’ calculations showed that a simulated shift in the due date from January to October would be associated with an average reduction in birth length of about 0.8 millimeters.

Compared to an average birth length of about 50 centimeters, this difference is very small. For individual children, such a difference would be neither visible nor medically relevant. Nevertheless, scientists attach importance to such findings. In large populations, even small changes can provide important clues to biological relationships. Furthermore, the results show that temperature stress could have measurable effects on developmental processes—even if these initially appear minor at the individual level.

From a public health perspective, this is of interest because, with climate change, more and more people will be exposed to longer and more intense heat waves. Small effects at the individual level could thus gain significance at the population level in the long term.

What Biological Mechanisms Might be Behind this?

The study itself cannot answer why higher temperatures are associated with shorter birth length. It merely demonstrates a statistical correlation. Nevertheless, the researchers discuss various possible biological mechanisms. The placenta could play a central role. It supplies the unborn child with oxygen and nutrients throughout the entire pregnancy. High temperatures can place a strain on the mother’s body and potentially cause changes in blood circulation. This could also alter the supply to the fetus.

Furthermore, it is known that heat stress can trigger various physiological responses. The body attempts to dissipate excess heat by increasing blood flow to the skin and boosting sweat production. These adaptive mechanisms could indirectly influence other bodily functions that are important for pregnancy.

Changes in fluid balance are also being discussed. At high temperatures, the risk of dehydration increases, especially if fluid intake is insufficient. Furthermore, hormonal changes, inflammatory responses, or metabolic adaptations could play a role. However, there is no clear explanation as of yet. Further studies will be necessary to better understand the biological processes.

Climate Change as a Challenge for Health Research

The findings are being published at a time when the health consequences of climate change are increasingly coming into focus. Austria has warmed significantly more than the global average in recent decades. Heat waves occur more frequently, last longer, and reach higher temperatures than they did just a few decades ago. While the effects of extreme heat on older adults, people with cardiovascular diseases, or chronic conditions are already well documented, research into how high temperatures affect pregnant women and unborn children has only intensified in recent years.

International studies have already described links between heat and preterm births, low birth weight, or pregnancy complications. The current Austrian study now expands this knowledge to include another possible aspect: the link between ambient temperature and birth length. In doing so, the study helps paint a more comprehensive picture of the health impacts of rising temperatures.

Greater Focus on Heat Stress During Pregnancy

The researchers view their findings as a reason to devote more attention to the issue of heat stress during pregnancy in the future. Study leader Thomas Waldhör emphasizes that the effects of heat on fetal development may have been underestimated until now. Especially in light of advancing climate change, it may become necessary to develop targeted prevention strategies for pregnant women. These could include recommendations on behavior during heat waves, improved medical counseling, or measures to protect particularly vulnerable population groups.

At the same time, the scientists caution against overinterpreting the results. The observed differences are small, and the study provides no evidence that high temperatures directly cause health damage. Rather, it represents an important building block in a growing field of research that examines the complex interactions between the environment, climate, and human development.

Conclusion

The new study by the Medical University of Vienna, the University of Vienna, and the University of Calgary shows that higher ambient temperatures during certain phases of pregnancy may be associated with a slightly shorter birth length. The study was based on data from more than 1.2 million singleton births in Austria over a 30-year period. Even though the observed effect is small, the results underscore the importance of environmental factors for the development of unborn children. They also show that the health consequences of climate change extend far beyond the known risks of heat waves and may begin even before birth.

The researchers therefore call for further studies to better understand the biological causes and develop potential protective measures for pregnant women. With increasing global warming, the question of how heat affects early childhood development is likely to gain further significance in the coming years.