Exposure to high heat and humidity during pregnancy can stunt children’s height as they get older.
Heat exposure alone can stunt growth, but researchers found that adding humidity compounds the problem considerably.
The body sweats, and that sweat evaporates as a cooling mechanism when temperatures climb.
But in humid conditions, that sweat cannot evaporate, meaning the body cannot cool itself effectively. This drastically increases the risk of heat stress, making hot-humid conditions far more dangerous than heat alone.
The latest study shows that exposure to hot-humid days during any trimester of pregnancy reduces child height-for-age up to four times more than exposure to high dry heat alone.
Core temperature is most affected in hot-humid conditions, by compromising infant health at birth, such as through prematurity or low birth weight. Prenatal heat exposure also has lasting effects on long-term health and development.
Heat is especially dangerous during pregnancy because hormonal and metabolic shifts disrupt the body’s cooling system, making pregnant women more sensitive to heat and heat-induced dehydration that can induce early labor.
Together, these factors increase risks of heat stress, pregnancy loss, stillbirth, preterm birth and low birth weight.
According to new research, experiencing hot and humid weather at any point during pregnancy can impair a child’s growth up to four times more severely than exposure to dry heat (stock)
Under future climate scenarios, the researchers estimate that hot-humid extremes could push 3 to 3.7 million more children into stunted height, a burden 2.7 to 3.3 million greater than if only dry heat were considered.
This study used a fine-scale, quasi-experimental design to analyze prenatal heat exposure for approximately 200,000 children across South Asia.
The researchers linked children’s health data to detailed, location-specific weather data, specifically tracking exposure to two problematic temperature thresholds during pregnancy: days exceeding 35°C (95°F) in maximum temperature and, more critically, days exceeding 29°C (84.2°F) in maximum wet-bulb globe temperature (WBGTmax), a metric that combines heat and humidity.
By comparing children within the same communities and birth months across different years, the analysis accounted for seasonal weather trends.
The researchers found that a one-standard-deviation increase in the number of these hot-humid days during the critical third trimester was associated with a 5.1 percent decrease in a child’s height-for-age score.
In contrast, an equivalent increase in exposure to days with a maximum temperature (Tmax) above 35°C (95°F), dry heat, resulted in only a 1.3 percent decrease, indicating that the growth-stunting effect of humid heat was nearly four times that of dry heat.
The finding underscores that the critical hazard is not high temperature alone, but the combination of heat and humidity, which severely impairs the body’s ability to cool itself and imposes greater physiological stress on both the pregnant woman and the developing fetus.
An ever-warming planet and a rise in the number of previously temperate climates being reclassified as sub-tropical, including New York City, paves the way for more children falling short of their full-height potential. Hot air holds more moisture, so a warmer world is a more humid one.
Gulf states, including Florida, Alabama, Mississippi, Louisiana and Texas, as well as South Atlantic states, including Georgia, South Carolina, North Carolina, Virginia and Maryland, are designated as having subtropical climates characterized by warm, humid summers and mild winters.
A growing number of states are seeing warmer, more humid summers, with the East, Midwest and South seeing significant increases in dangerous humid heat days, impacting health and causing record-breaking heat index values.
A Washington Post analysis of dew point since 1979 found that over 120 million people across 1,500 counties in the US just endured one of their three most humid summers ever recorded.
The regions most affected included large portions of the Mid-Atlantic, Southeast and Plains.
The dew point, which indicates the temperature at which moisture in the air begins to condense, is a direct measure of humidity and captures the feeling of oppressive, sticky heat that many experienced.
Several studies in recent years have identified measurable harms done to fetuses when pregnant women are exposed to high heat and humidity, finding that exposure can increase the risk of a baby being born too early, weighing too little or with potentially severe heart defects.
By the 2025 to 2035 period, specific heart conditions are projected to rise sharply in certain regions: heart abnormalities in the structure of the major arteries leaving the heart could increase by about 34 percent in the South.
Atrial septal defects, holes in the wall between the heart’s two upper chambers, which can cause long-term strain and complications, could jump nearly 39 percent in the Northeast.
Studies have also found that outside temperature also influences a baby’s risk of being born with birth defects of the brain, spine and spinal cord.
These defects can cause spina bifida, where the spinal column does not close completely during early pregnancy, damaging the spinal cord and nerves, leading to varied symptoms like paralysis and bladder or bowel issues and problems affecting the musculoskeletal system.
