Hot and Humid Weather During Pregnancy Poses Far Greater Risks to Child Health Than Heat Alone
Hot weather during pregnancy has long been recognized as a health concern, but new research shows that heat combined with humidity is far more dangerous than previously understood. A recent study published in Science Advances reveals that when pregnant women are exposed to hot, humid conditions, the negative effects on their children’s health can be dramatically worse than exposure to high temperatures alone. The findings suggest that scientists, doctors, and policymakers may be underestimating the true health burden of extreme weather, especially in regions where humidity is consistently high.
The research was conducted by a team at the University of California, Santa Barbara (UCSB), led by doctoral researcher Kathryn McMahon under Professor Kathy Baylis from the Geography Department. The study focused on South Asia, a region with a massive population, high baseline humidity, and increasing exposure to extreme weather events driven by climate change.
Why Humidity Changes Everything
Most people are familiar with the idea that humid heat feels worse than dry heat. There is a solid biological reason for this. The human body cools itself primarily through sweating, which relies on evaporation. When the air is already saturated with moisture, sweat evaporates more slowly, making it harder for the body to release heat. As a result, heat stress can occur at much lower temperatures when humidity is high.
To capture this effect accurately, the researchers did not rely solely on air temperature. Instead, they used a metric called the Wet-Bulb Globe Temperature (WBGT). Developed in the 1950s, WBGT accounts for air temperature, humidity, radiant heat, and airflow, making it a far more realistic measure of how heat affects the human body.
In this study, extreme heat was defined as days with maximum temperatures above 35°C, while extreme hot-humid conditions were defined as days when WBGT exceeded 29°C. These thresholds were chosen because they occur at similar frequencies in South Asia, allowing for a fair comparison between heat alone and heat combined with humidity.
How the Study Was Conducted
The researchers combined large-scale Demographic and Health Surveys (DHS) with high-resolution daily weather data from the Climate Hazards Center at UCSB. The DHS data included health information for tens of thousands of children under the age of five, spread across nearly 30,000 geographic clusters in South Asia.
The key health indicator examined was height-for-age, a widely used measure of chronic child health and long-term development. Children who are significantly shorter than the average for their age are considered to have experienced stunting, which is associated with lifelong health, cognitive, and economic disadvantages.
By linking each child’s location and approximate prenatal period with daily temperature and humidity data, the researchers were able to estimate in-utero exposure to extreme heat and hot-humid conditions across different stages of pregnancy.
The Results Were Striking
The findings clearly showed that humidity greatly amplifies the damage caused by heat. Exposure to extreme heat alone had a negative effect on child growth, but the impact of hot and humid conditions was about four times worse.
Specifically, the study found that a one-standard-deviation increase in hot-humid exposure during pregnancy was associated with a 13% reduction in height-for-age. In contrast, a similar increase in exposure to extreme heat alone resulted in only about a 1% reduction. The difference was especially pronounced during the third trimester, when the combined stress of heat and humidity had the strongest effect.
These results strongly suggest that studies and health guidelines that focus only on temperature are missing a critical part of the risk.
Why Pregnancy Is a High-Risk Period
Pregnancy already places significant stress on the body. Pregnant women generate more internal heat due to increased metabolic activity, carry additional weight, and experience hormonal changes that can impair heat regulation. When these factors are combined with high humidity, the risk of overheating rises sharply.
The study identified two particularly vulnerable periods: very early pregnancy and very late pregnancy. Early in pregnancy, the fetus is undergoing rapid and delicate development, making it especially sensitive to environmental stress. Late in pregnancy, the mother becomes more vulnerable to heat stress, which can trigger premature labor. Babies born early often face developmental challenges that can persist throughout childhood.
One especially concerning finding is that the period around conception, and even just before pregnancy begins, may also be risky. Many women may not yet know they are pregnant during this time, which makes targeted protection more difficult.
Addressing Data Limitations
The authors were careful to acknowledge the study’s limitations. Exact birth dates and precise pregnancy lengths were not available, which means the researchers could not fully account for the effects of premature birth on growth outcomes. However, they tested their results using multiple alternative thresholds and modeling approaches.
Across all these checks, the main conclusion remained the same: humidity dramatically worsens the impact of prenatal heat exposure. The team also examined whether increased fetal loss or infant mortality could be skewing the results and found no strong evidence that these factors were driving the observed patterns.
Why This Matters for a Warming World
The implications of this research are far-reaching. Many of the world’s most densely populated regions lie along coastlines, rivers, and lakes, areas that are naturally more humid. As of 2018, roughly 38% of the global population lived within 100 kilometers of a coast, with even more people living near inland water sources.
South Asia alone is home to over 1.7 billion people, and climate models suggest that hot-humid extremes will become more frequent and severe in the coming decades. Under high-emissions scenarios, millions of additional children in the region could experience growth stunting by mid-century.
Importantly, much of the existing research on extreme weather focuses on mortality, but this study highlights how many more people are harmed without dying. Chronic health impacts like stunting carry long-term economic and social costs, often reinforcing cycles of poverty and poor health across generations.
What Can Be Done
The researchers emphasize that the situation, while serious, is not hopeless. Small interventions can have large benefits. Improved heat forecasting systems that track WBGT rather than temperature alone could provide earlier and more accurate warnings. Public health messaging focused on hydration, rest, shade, and cooling strategies could significantly reduce risk for pregnant women.
The Climate Hazards Center is already working on heat early-warning systems in vulnerable regions, including refugee camps in East Africa. Other research efforts are exploring how heat interacts with additional stressors like air pollution, and how different groups may face unequal risks.
Understanding WBGT and Heat Stress
For readers unfamiliar with WBGT, it is worth emphasizing why this metric matters. Unlike the commonly reported “feels like” temperature, WBGT is grounded in human physiology. It has long been used in occupational health, sports medicine, and military planning to prevent heat-related illness. This study shows that WBGT also has critical relevance for maternal and child health, especially in a changing climate.
Research Reference
Does humidity matter? Prenatal heat and child health in South Asia, Science Advances (2025)
https://www.science.org/doi/10.1126/sciadv.adx3010
