Prenatal exposure to hot, humid conditions linked to impaired child growth

Extreme heat is a well-recognized health risk during pregnancy, but new research suggests that humidity substantially amplifies its long-term effects on child health. A study published in Science Advances found that prenatal exposure to hot, humid conditions was associated with significantly worse growth outcomes in children than exposure to high temperatures alone, particularly in South Asia, where heat and humidity frequently coincide, and rates of childhood stunting remain high.^1,2

In the analysis, researchers from the University of California, Santa Barbara, evaluated how prenatal exposure to extreme heat and humidity affected early childhood growth, using height-for-age as a marker of chronic health status. The investigators found that accounting for humidity quadrupled the estimated effect of extreme heat exposure on child growth, suggesting that temperature-only assessments may substantially underestimate climate-related health risks.

“Exposure to hot, humid conditions in-utero is dangerous for child health, and more dangerous than just hot temperatures alone,” said lead author Katie McMahon, a doctoral student under Professor Kathy Baylis in the Geography Department.

Measuring heat stress beyond temperature alone

Human thermoregulation relies on the evaporation of sweat, a process that becomes less effective as humidity rises. When evaporation is impaired, body heat accumulates, increasing the risk of heat stress even at lower air temperatures. To better capture this physiological burden, the researchers used the wet-bulb globe temperature (WBGT), a composite metric that accounts for air temperature, humidity, radiant heat, and airflow.

“That’s why the wet-bulb globe temperature was invented in the 1950s,” McMahon said. “And when evaporation can’t happen, then cooling can’t happen. All that heat builds up in our bodies, causing heat stress.”

Using WBGT allowed the researchers to distinguish between dry heat and humid heat exposure during pregnancy. Extreme conditions were defined as daily maximum temperatures above 35°C (95°F) or WBGT values above 29°C (84°F), thresholds that occurred with similar frequency across the study region.

Study design and population

The study linked child health data from the Demographic and Health Surveys with high-resolution daily weather data produced by the Climate Hazards Center at UC Santa Barbara. The final sample included nearly 200,000 children younger than 5 years from Bangladesh, India, and Nepal.

Height-for-age was used as the primary outcome, a widely accepted indicator of chronic undernutrition and long-term health in early childhood. The researchers examined heat and humidity exposure across four prenatal periods, including the three trimesters of pregnancy and the three months before conception.

After adjusting for demographic, seasonal, and geographic factors, the authors found that prenatal exposure to hot, humid conditions had a far greater negative association with child growth than exposure to high temperatures alone.

Magnitude and timing of effects

The strongest effects were observed during early and late pregnancy. According to the analysis, a one–standard deviation increase in hot, humid exposure during the third trimester was associated with an approximately 5% reduction in height-for-age, compared with a 1% reduction associated with extreme heat alone. When exposure occurred across multiple trimesters, the cumulative effect was substantially larger.

“The worst times were very early and very late in pregnancy,” McMahon explained. “At the beginning of pregnancy, the fetus is very vulnerable, while at the end of pregnancy, the mother is more vulnerable.”

Physiologic changes during pregnancy, including increased metabolic heat production and altered hormonal regulation, may increase susceptibility to heat stress. High humidity further impairs the body’s ability to dissipate heat, increasing the risk of adverse maternal and fetal outcomes such as preterm birth.

Heat exposure just before conception also appeared to carry risk. “A woman herself may not know she’s in this critical time for her future child, or that she’s pregnant at all,” McMahon said.

Broader implications for climate and public health

The findings have important implications for climate risk assessment and public health planning. Temperature-based metrics alone may overlook densely populated humid regions, including river valleys and coastal areas.

“These are literally the cradles of civilization,” said co-author Chris Funk, director of the Climate Hazards Center. “And so they’re the most densely populated places on the planet.”

Using climate projections, the authors estimated that under a high-emissions scenario, more than 3 million additional children in South Asia could experience growth impairment by 2050 if exposed to projected levels of hot, humid conditions. Estimates based on temperature alone would identify only a fraction of this burden.

“Extreme weather harms many more people than it kills,” McMahon said. “This affects our calculus of the cost of climate change for human health and society overall.”

Opportunities for intervention

Despite the seriousness of the findings, the authors emphasized that improved measurement and early warning could support mitigation. Tracking WBGT rather than temperature alone may help guide public health messaging, clinical counseling, and early warning systems for pregnant individuals during heat events.

“It’s important to note that many vicious cycles are virtuous cycles run in reverse,” Funk said. “Small interventions could yield dramatic, far-reaching effects.”

The researchers noted that further studies are needed to better define vulnerable populations and evaluate targeted interventions, particularly in regions where extreme heat and humidity are expected to intensify in the coming decades.

References

1. University of California - Santa Barbara. Hot, humid conditions during pregnancy undermine child health much more than heat alone. Eurekalert. December 19, 2025. Accessed December 22, 2025. https://www.eurekalert.org/news-releases/1110743
2. McMahon K, Baylis K, Sweeney S, Funk C. Does humidity matter? Prenatal heat and child health in South Asia. Science Advances. 2025;11(51):eadx3010-eadx3010. doi:https://doi.org/10.1126/sciadv.adx3010